Well, I wouldn't necessarily suggest this for all campaigns, but in the ones we run, we just run spellpoints by the stat used for casting. Each class has different ways of applying these points.
Sorceror: (modifyer )+2
Cleric: (modifyer) +1 from domains
Wizard: (modifyer) +1 from specialty school
Bard: modifyer, no bonus/penalty
Ranger/paladin: Modifyer -1, minimum 1 (I.E. a paladin with 12 wis has 1 spell point per lv, 14 wis is still 1 spell point per lv, but 16 wis is 2 spell points per lv)
Thus, a sorceror with 16 CHA at level 1 can pick 5 levels of spells, this case only 1st level spells (or 0th lv spells, which cost 1/2 a point to cast or learn), so they can pick 5 of them. The sorceror also has 5 spell points a day for casting purposes, so can cast 6 of his/her lv1 spells a day. Upon reaching 2nd lv, the sorceror will pick another 5 1st level spells, and now has 10 spell points a day, and thus may cast 10 1st lv spells before having to rest.
Now, at 4th lv, the same sorceror has access to 2nd lv spells. The sorceror can now pick 5 1st lv spells, 2 2nd lv spells and 1 1st lv spell, or any combination, so long as 5 levels of spells are used. The sorceror now has 20 spell points, and thus may cast 10 2nd lv spells, 20 1st lv spells, etc.
For further clarification, if the caster has 30 spell points, and casts a 3rd level spell, they now have 27 spell points, and if they cast a 2nd lv spell after that, they have 25 spell points. Casting any spell drains from the caster's spiritual/mental energies overall, rather than the more 'memorized' approach to casting. If a caster exerts his/herself on casting strong spells, they would not be able to cast very many, if any lower level spells, as they would simply get more and more exhausted as they cast more spells.
Also, there is no need for memorization. All casters cast as a bard or sorceror, in terms of preparation. However, all divine casters still know all spells that they are allowed to cast automatically. Only arcane casters have to pick their new spells known each level.
Special rules apply, however:
1) If the caster hasn't enough in their modifyer to learn/gain a new spell from a higher spell level, the caster gains as many spell points for the purposes of *learning* as their highest spell level, but still gains as many spell points for that level as they normally would for casting (I.E., at spell level 8, that same sorceror gains 8 points for learning, but still adds 5 points to their casting pool).
2) Seems too powerful, right? Well, there is a price. All casters using this system must focus on their chosen magic much more for the extra power and flexibility. The caster picks a school of magic (evocation, alteration, etc.) and that is their 'base' school. If it is a mage, they are automatically considered specialized in that school. They may only learn and cast spells from schools that are similar to their base school. Depending on how limited or abundant magic is in the campaign, this can range from only the 2 schools adjacent to your base school (going by the 2nd edition diagram of magic schools), to only *not* being able to cast from the opposition school of your base school and the 2 schools adjacent to the opposition school (I.E., your base school is alteration, so you can only cast alteration, illusion and enchantment, because those are the 2 schools adjacent/most similar to alteration). Exceptions apply for domain spells. How restricted magic school selection is, is dependant upon the campaign setting, and DM discretion. In addition, it is up to the DM if they will allow sorcerors to be slightly more broad in their schools than wizards.
3) When using metamagic feats, you cannot power up a spell to use points greater than your highest spell level. Thus the 'higher slot' rule still applies. (I.E. cannot power up a spell over 4 points if your highest spell level is 4)
If the player chooses to go with elemental magic, fire/water/air/earth, the opposition element may not be taken, but the other 2 may be (I.E. Fire as base school: can't cast water but can cast air and earth, but ONLY spells that involve those elements. No evocation, necromancy, etc.). DM judgement applies for certain spells not specified under the 4 main elements.
Thus, the overall system has a more simple spellpoint system (akin to videogame RPGs, pretty much), with much more flexibility in terms of being able to cast lower lv spells in favor of higher ones, and vice versa, and being able to cast any spell you have learned. However, you are limited, from somewhat to severely, in what spells you are allowed to cast. Thus, fewer casters would be alike. It's an extreme specialization form for spellcasting, really.
If you feel it is too powerful, you can always drop all of the modifyer's impact on spell points/selection by 1, but keep the minimum amount of spell points per lv as 1 regardless, so as not to hurt people without great stats, or rangers/paladins too much.
This method has worked quite nicely in our campaigns so far, so if you are willing to try a more simple (after all the rules are understood...hope I worded everything right), limited, but flexible method for spellcasting, you can try this out. Oh, and it's also better for lower level casters, as they can usually cast a decent amount of spells at low levels this way, and won't have to take the 'hide behind a rock after casting only spell allowed per day' approach to a low lv mage.