I currently only have a somewhat 'similar' rule for multiclassing: characters must meet certain requirements in order to multiclass into any given class, which are like the prereq's for PrC's, but less demanding.
Time must also be taken 'in game' to learn, study, train, practise, hone self-discipline, develop techniques, come to grips with new paradigms, and so on. This can be done during the course of adventures and such, but the progress might well be slower then. It's all 'played out', and this has worked well so far.
I also insist that if a player wishes to increase a character's skill rank in a particular skill, they must have utilised that skill consistently, or possibly in a new or significant manner. Reasoning: skills do not improve themselves. No complaints here either.
I'm not sure I like the idea of introducing training rules for levelling up per se, though - for my campaigns, I mean. If it works for you, or others, great!
I think I'll start insisting that feats and class abilities be learned however, either by means of self-training (for example, to move from Weapon Focus to Weapon Specialisation with a specific weapon), or courtesy of assisted training (for example, to learn Track).
Basically, I like the 'piecemeal' approach to training, rather than a 'whole-level-at-a-time' approach.
But I'm quite open to hearing any particular benefits or drawbacks either method might have.