Besides requiring spellcasters to multiclass, I've thought of some other ideas to make magic less accessible to anybody with a heap of gp and a shopping list or characters who simply survive. Here are a few that I cnosidered. They're not meant to be taken en toto, just some things I've thought about.:
1. Make the highest spell level available to PCs 5th level. Anything beyond that is really the domain of very powerful creatures such as dragons and extradimensional entities and deities.
2. Instead of the automatically successful casting of spells that is d20 magic, make casting a level check dependent upon the appropriate casting attribute and other modifiers. Of course, they wouldn't be called level checks. In my own rendition of the system, wizards and sorcerers had a casting check while clerics and paladins had a faith check. I introduced other elements that gave flavor while also using the mechanics (example: clerics can perform devotional activities like fasting or abstinence which heightens their connection with their deities gives a bonus to faith checks).
3. Replace sorcerers with psions (still Charisma-based use of powers, though) or make them substantially different from a wizard with the Eschew Materials feat. I considered giving them spells along a certain theme; their talent, if you will. A seer is different from a fire mage, who is in turn different from an enchanter. They would be one-trick ponies, but this is more to emphasize that magic would enhance and assist, not replace, their other abilities. Unlike wizards, their powers are less predictable when they use them (or even if they feel a little under the weather or pissed off). They somtimes do things a little too well, or do it just right in the wrong direction.
4. Wizards do not automatically gain new spells. As with any rigorous academic activity, they have to hunt down the correct information and try it out themselves. There are no wizards' guilds or magic academies, so learning from other wizards may be adventures in themselves. Requiring Spellcraft and/or Knowledge (arcana) ranks to learn spells of a certain type or level seems like a good start.
5. Clerics and druids would get a major overhaul, but for the time being, I'll talk about their spells. Clerics have to serve a deity. It needn't be the deities listed in the PHB. Initially, clerics should only be able to cast domain spells. All their other boons, such as spontaneous spells, would be spread out more evenly across levels. The spontaneous spells given would be granted based upon the domains the cleric has chosen. I may decide to make druids a PrC or use that as the name for any divine caster who reveres a nature deity.
6. Replace ranger spellcasting with bonus feats and/or class abilities appropriate to the class.
7. Make paladins more like the martial arm of religious ideals and organizations than the all-around-good-guy thing that they are. I may just get rid of them and label any divine caster who focuses on defending the faith, smiting the enemies of their deities or ideals, and leading the people to righteousness as paladins.
8. Give non-casters unique abilities that allow them to hold their own without needing tons of magical items. Make sure said options account for non-tank fighters and non-thief rogues.
9. Disallow monks, give more options for unarmed combat (such as a feats that increase unarmed damge or simulate flurry of blows but can be used with any light weapon), or get rid of the Shaolin kung-fu mystical mumbo jumbo stereotype.
10. Increase the utility of skills and feats. Giving certain class abilities skill requirements, making skill synergy work based on actual ranks in the skill (a +1 bonus for every 5 full ranks seems good), importing feats that are useful and somewhat balanced seems like they could work, and reworking the class/cross-class system are just a few things I could try.
11. Magic items would be by-and-large skill-boosting items or items that bestow feats instead of the blatantly obvious magical items. Of course, there would be exceptions. Very expensive, hard-to-find exceptions.
12. Class-based defense bonus so that characters could avoid being hit (especially at high levels) without needing magical armour. I'd base it on actual combat skill as opposed to the base reflex save. I'd likely use Unearthed Arcana's Armour as DR in conjunction with it.