Here is a semi-elaborate plan for how I always idealized the way multiclassing should work.
Given:
1) It is reasonable that a character should be able to learn a new class at any point in the game.
2) It is reasonable that picking up a new class represents a significant paradigm shift in that character's way of thinking (i.e. it is challenging to learn all the skills of a new class in a short period of time).
3) If you do not continue to practice and develop your skills in your old class, you will eventually lose those abilities (use it or lose it; IRL, the longer you go without doing something, the more rusty that skill set becomes).
So we from the 3e and PF crowd like the idea that anyone should be able to learn a set of skills at any point in time because it feels organic, which is a more permissive form of 1e/2e dual-classing. However, it is somewhat unrealistic to just allow a character who has spent five levels as a wizard to just happen to pick up all the starting skills of a 1st-level fighter at 6th-level. In the game, it is assumed that this training takes 1d4 years (this is inferred from Table 6:4 of the 3.5 Player's Handbook, which is preserved for the most part in PF as well) for a young fresh human mind to learn these skills. We have a slight problem then. The player wants these abilities (the weapon proficiencies, armor proficiencies, shield proficiencies and the bonus feat) of the fighter and wants them now. But it doesn't seem quite right, realistic, or reasonable to give them immediately when the 6th-level fighter who began at level 1 spent 1d4 years training all those skills. Even more could be said of the 5th-level fighter would decides at 6th-level he wants to become a wizard. This training takes the 1st-level human wizard
2d6 years! Varsuvius makes an
excellent case.
3e/PF fans are usually also big fans of multiclassing, but when it is so incredibly easy as to simply say "I think I shall take my next level as wizard," it creates a disturbance in the suspension of disbelief by pushing the absurdity to a point which is actually contrary to the defined spirit of the game (that training as one class or another often takes years of study and training). I have a couple good solutions (which I have used in 3e games before) to this dilemma and I shall lay them out individually.
Solution 1: Inherent Benefits Vs. Level-Based Benefits
Perhaps the best notion is to separate the game mechanics of that training period from the mechanics of the level-based benefits of the class. I will use 3e as my template for this solution, but it is only an example; the concepts can be applied to D&DN. In 3e, a 5th-level wizard who wants to take a level of fighter does not gain any of the weapon, armor, or shield proficiencies inherent to the fighter class. That ship has already sailed. Note the 3e system which is well-liked by many already denies a substantial number of 1st-level skill points to multiclass characters. At 1st-level a character gains four times as many skill points as he or she would in 2nd-level and onward. So there is already some precedence for this in the rules. Since the fighter's bonus feat at 1st-level is also out of step with the bonus feat progression of one bonus feat every two levels we might also consider that 1st-level bonus feat an inherent benefit of training. Furthermore, the fighter receives a special bonus of +2 to his Fortitude save at 1st-level which might also be considered an inherent benefit of training. So what would the wizard gain by taking a level of fighter then? Quite simply, the wizard would only receive a fighter's hit dice, skill points, base attack bonus, and saving throw advancement (but not the +2 inherent to Fortitude, so essentially it is +1/2 Fort, +1/3 Ref, and +1/3 Will; this would go well with the fractional BAB and saving throw bonus rules from Unearthed Arcana to be a little more fair). It doesn't seem like a lot. The wizard will have to take at least two levels of fighter to get a full bonus point to Fortitude and a bonus feat. But this is very reasonable if we agree with the above assumptions. Suddenly becoming proficient with every simple and martial weapon, every suit of armor, and every shield in the game is the equivalent of many feats (which are poor feats, taken individually, but the point remains, even if they were broken up into weapon groups a la the Unearthed Arcana weapon groups variant, it is still many feats clumped in one level). So a wizard would need a very good reason to want to take that level of fighter (perhaps a need for more hit points, higher BAB, or plan to advance further to gain bonus fighter feats). The wizard would not be able to simply cherry-pick a whole suite of abilities with one level.
What would the converse situation look like? A 5th-level fighter wants to gain a level of wizard. What are the wizard's inherent abilities? The fighter already has all the wizard weapon proficiencies, so what does a typical wizard gain in one level? The wizard gains 2 spells of any level he or she can cast, 2 + Int skill points per level, advancement in familiar abilities (if any), and 1/5 of the way towards a new wizard bonus feat, +1/2 BAB, +1/3 Fort, +1/3 Ref, and +1/2 Will. That is exactly what the fighter would get. The fighter would not get the Scribe Scroll feat, as it is out of step with the progression of other wizard abilities, nor would the fighter get 3 + Int spells and all cantrips to begin a spellbook (in fact since read magic is a required wizard spell, a fighter should probably be required to choose read magic as one of his or her 2 spells at 1st-level), nor would the fighter get +2 to Will. Having 3 cantrips per day at 1st-level might be considered to include an innate 2 cantrip bonus as well so the fighter only receives one cantrip and one 1st-level spell per day (but continues to progress in spells per day at the normal rate).
This process can be done for every single class, singling out the benefits that are out of step with normal advancement and discarding these from advancement because they are inherent in the years of training that is required for the class. Some other examples of inherent benefits might include: a barbarian's fast movement, a bard's ability to fascinate, a cleric's ability to turn undead, a druid's nature sense, a monk's bonus feat, a paladin's ability to detect evil, a ranger's Track feat, a rogue's ability to find traps, 2 of the sorcerer's bonus cantrips (I actually think the sorcerer is one of the least incredulous multiclassing options because it is not front-loaded in any way aside from the +2 to Will). Does this remove several of the iconic abilities from classes? Of course it does, but then, if your character did not spend years training in that class, why should you expect to have those all of those abilities from the start? If a character really wants to have those abilties, the character will have to spend the same amount of in-game time training that all the other characters who start at 1st-level in that class have to take. This means an elf wizard who chooses to multiclass as a fighter would likely never gain all the weapon proficiencies of the fighter in the time it takes to complete the campaign, but a human conceivably might. But this is realistic within the milieu of the game and is actually somewhat balanced, as many of the non-human races have significant advantages over humans (and the half-elf and half-orc are both considered fairly weak choices for most characters, both of which have short training times). You might even choose to place more emphasis on favored classes, allowing characters who multiclass into their races' respective favored classes receive more or all of the inherent benefits of a class.
The power gamers are probably screaming in agony at their screens right now, but that is because the 3e multiclass system was ill-conceived by heavy front-loading of so many of the classes (and I won't even get into the problems with prestige class and splat book base class cherry-picking). It practically invites characters to multiclass because it is more advantageous for practically all characters to multiclass at least a little (unless a spellcaster with access to 9th-level spells) than it is for them to stick with only one class. That's a problem. 3.5 went a little ways towards reducing the front-loading, but it didn't go far enough, as multiclassing is still rampant, if not the preferred method of playing the game. PF went a little further, but then added a number of other abilities to many classes, which was counter-intuitive, as the glut of multiclass characters in PF is still as evident as it was in 3.5. This solution is both more realistic within the milieu, more balanced, and more representative of a class-based game. Your first class is your where your strengths lie. If you want to diversify your character's abilities by multiclassing, you gain just as much as another character who is already leveling in that class would get, which is incentive to keep to your primary class for the most part. Again, this is realistic within the milieu if we accept that training in a new class represents a significant paradigm shift in the character's way of thinking. As a real-life parallel example, an accountant of 20 years who decides to become a lawyer is not going to have as much use for his accountant skills and he will have to spend several years in school training as a lawyer.
Solution 2: 1e/2e/3e Gestalt/4e Hybrid-Based Model
This solution is less than ideal for some because it requires the splitting of XP across multiple classes. For DMs who do not like to track XP (I am one of them), this idea may not appeal to you, but you can still estimate it. This solution relies on several abstractions. First of all, the key to this system is that the more classes you have, the slower you level up. This system is more permissive in allowing front-loaded abilities because it is relying on the fact that your overall level is increasing more slowly and the fact that you receive fewer benefits per level to balance that out. Second, the system assumes that you must split your XP evenly among all classes for the sake of simplicity. Variants where a character may choose to focus on one class more than another are possible, but would require some tweaking. Third, it does not allow for much flexibility if one character decides to completely stop focusing on one class and move over to another, but it still allows for that possibility, at a penalty. On the other hand, it does allow for a more fluid ability of characters to pick up a new class instead of only allowing it when they reach a new level.
This is how it works. I will use 3e as a model for the purpose of explanation since D&DN advancement is a little closer to 3e than 4e. At any time, a character may begin leveling up in a second class. At this time, the character gains all of the abilities of a 1st-level character in the class, with a few caveats. (A DM might want to phase some of these abilities in slowly. I like the 3e UA weapon groups or 4e weapon proficiency system as an example of phasing in these sorts of things. You could allow one weapon group per level until the character has all the weapon groups of that class, as an example.) The main caveat is that the character gains the greater of any abilities between the two classes. This applies to attack bonus, saving throw bonus (both of which aren't as relevant with D&DN bounded accuracy, but the concept is the important thing to keep in mind), skill points, and so on. Think of 3e UA gestalt characters. A 1st-level wizard/1st-level ranger has the HD, BAB, Fort and Ref progression of a ranger, the Will progression of a wizard, and gains 6 + Int skill points per level (the higher of the wizard's 2 + Int vs. the ranger's 6 + Int). The same still holds true of a 9th-level wizard who decides to also become a 1st-level ranger. The character would use the higher of the wizard's 9th-level BAB, Fort, Ref, and Will (which are all higher than that of a 1st-level ranger), but would gain 16 skill points for being a 1st-level ranger (the difference between 2 x 4 and 6 x 4) and 4 skill points for each additional ranger level. The character would also gain the difference in hp between a d8 and d4. (This might require due diligence of tracking of hp rolls if you use rolled hp, so this solution works best if you use static hp bonuses per level). At 1st-level, the ranger level would add 4 hp to the wizard (the difference between a maximum d4 and a maximum d8), then at each additional level the character would compare his ranger roll to his wizard roll and use the ranger roll if it is higher (or if using static hp, gain the difference between the two). Again, the DM might want to phase this in (perhaps 8 skill points per level for the first four ranger levels and 4 skill points per level after). The next caveat is that all additional classes (3rd, and 4th etc.) follow the same rules, so there is a sense of diminishing returns. If you are already a 10th-level wizard/3rd-level ranger, a level of fighter isn't going to get you a whole lot besides a small number of hit points, a bonus feat, and some armor and shield proficiencies which you may or may not want to use.
In the example above, the wizard class will always be your most dedicated class, because you started as a wizard first. And because you are splitting XP evenly between all classes, it will continue to be the highest, although the other classes will eventually get close. You are required to continue training in your wizard skills because of the use-it-or-lose-it philosophy. It is the idea that skills unpracticed become rusty. Much like a professor of biology who does not keep up with the current scientific literature loses his proficiency in the language and the state of the current science, so the wizard would lose his proficiency in spellcasting and his familiar if he were to devote all of his XP earned to his new ranger class. Well what if a character does not wish to spend any of his XP on his old class and wishes to level up his ranger class exclusively from that point? How do we represent this loss of proficiency?
The player may indeed decide to advance exclusively as a ranger, but for every 2 points of experience he is earning as a ranger, he loses 1 point of experience as a wizard. This will never result in the loss of levels as a wizard, merely the loss of XP in the wizard advancement. The skills the wizard has learned remain intact, and especially if he continues to use them, those skills remain as good as they ever were. But if the wizard/ranger is not diligent in continuing his studies as a wizard alongside being a ranger, his proficiency with learning the ways of a wizard will be set back. If he ever decides to go back to studying as a wizard, he will have to make up for lost time, as it were. But this does mean that the wizard/ranger may have the ability to focus on the ranger class long enough to get the two equal in experience points so that the character may proceed to level up both at the same rate. This penalty is a necessary rule because it both helps preserve the suspension of disbelief by keeping within the spirit of the milieu and gives the player incentive not to cherry pick many different classes, which is somewhat unplausible, even within a fantasy setting (unless you envision humans and elves in a fantasy setting as also having super-human abilities to multitask, in which case you can discard this rule, but it comes at a loss of game balance).
This is a rather elegant way to go if you don't mind separately tracking XP. We used to do it for decades back during the 1e/2e days and few people seemed to mind it then. If you don't like to track XP (I often do not) and would rather level up the player characters when you feel it is appropriate to the story, by all means go right ahead. You will just need a fast and dirty method to estimate things. Perhaps a simple rule of 2 levels in the new class for every 1 level in the old class until the new class is within 2 levels of the old class. So, for example, the 9th-level wizard who picks up a level of ranger, will gain his second level of ranger before he gains his 10th level of wizard, and his third and fourth level of ranger before he gains his 11th level of wizard, and so on until the ranger level is 2 levels below. The overall character level (remember this is essentially a gestalt or hybrid character) however should probably fall behind the rest of the party by 1 level for balance, or 2 levels if the character takes a third class, and so on.
Summary
These are two rules which I have tested and which have generally been well-received by players. They are a bit difficult to understand at first, and traditional power gamers might frown upon them because it requires them to learn a new system in order to power game it, but tell them to think of it as a new challenge and a new experience rather than a burden. I think if one of the two above rules is taken as part of the architecture for multiclassing in D&DN it will work exceptionally well. I have put much thought and received input from many people over the course of many years and I think either one would be a good direction to go. The second solution in particular has the advantage of being based on systems that have been used before and which many people understand well, although I consider it a little less realistic. The first solution is actually my preferred method, but the system has to be designed with it in mind so that you do gain something of value at every level. Some classes don't lend themselves well to this because of dead levels (i.e. the 3e fighter), so it really must be something that is built into the system. There would need to be a clear delineation of inherent bonuses which can only be earned at 1st-level (and at the DM's option, with the appropriate in-game training time if it is a long-term campaign) and level-based bonuses for each class, and these would need to be balanced such that there were no dead levels so that each class grants something roughly equivalent in value at each level.