I think there's a problem with multi-class spellcasters. The power of their spells are too weak, especially in high level play. The game effect is virtually insignificant as high level opponents can easily resist such spells, and buff/heals aren't really worth it. A solution I've been toying with is to treat spellcasting like a cross-class skill. As you advance in the non-casting class, the effective caster level and things tied to it (damage, duration, etc) would continue to increase but at half the level (rounded down). For example, a 20th level character made up of 19 Fighter levels and 1 Wizard level would be able to cast the few spells he knows as a 10th level caster. The character isn't learning anything new as a caster, but is still practicing and improving the few things that he does know.
I'm thinking this could also be expanded to all level-based abilities. Again, not gaining any new abilities (or new spell levels), but to increase the effectiveness of the abilities he has.