I've always liked the idea of giving my PCs mentors. These are the long-bearded old loons who trained your eager young mage. The one-armed fencing masters who secretly taught your warrior princess swordplay under a disapproving queen’s nose. The Ancient Masters who bestowed the power of the stunning fist on your journeyman monk.
When I'm on the other side of the GM screen though, I find that it's hard to give the idea the time it deserves. What I mean is that, if a player presents me with an NPC that they want to work into the game, it sometimes doesn't mesh with the flow of the campaign. For example, if that journeyman monk expects to head back to the monastery to meditate before level up, that's going to be tough to pull off in a globe-trotting sort of game.
So here's my question to the board: have you seen a master/apprentice relationship pulled off well in a game? How did the GM in the situation manage to keep this wise-old-NPC relevant as the game progressed?