Are there enough PrC's to require all characters to take one?

[Galadare]

I think if you enforce pre-requisites, and failures when PCs try to do something they can't, then your ultimate goal will be accomplished.

I think smetzger has the right idea.

I don't know how you run your game but you might also try making your players use the mechanics for their skills or feats even if they role-play it out well. You could then reward their role-playing with a bonus to thier check.