In more serious campaigns, I demand my players to have someone better graduated to teach them, and role-play character training between adventures. I don't do that on each level, though; as someone pointed earlier, it gets old really fast. If you care to train when you have the opportunity, you'll level up when you get enough XP.
Finding a good tutor also rewards the players in my campaigns. I like to award both XP and other extras (such as secret fighting techniques or spells) for players when they do the homework and try to find the best masters in whatever they want to train in. Sometimes I even create sub-quests that players must accomplish in order to train in what they want to.
It does take a lot of time out of typical adventures, but we find this kind of thing to be as much a legitimate part of campaigning as dungeon crawling and demon hunting.
Cheers,