As I've mentioned before, I'm not fond of 3e-style "I'm a wizard, but I'm going to be a monk for this level" style class-switching. But I do think there should be room for some kind of multiclassing. If the character classes really represent years of training and study, then, how are we to model that in the game? (Presuming we aren't simply using the 1e AD&D rules.)
I like two options: one, a character can start out with two classes, representing an extraordinary background; I would require such a character to have at least 13 in the prime attributes of each class. Two, characters can gain another class later on, but I would limit it to one every 4 levels past first, and the player has to pre-declare what their character is studying to become.
In any case, once the character has multiple classes, experience must be split between them.
Since I like to start 1st-level characters with max hit points, the question becomes, which die to use? If you're a "nice" GM (and I try not to be, but who am I kidding?), you might give the best of the two. But you could also roll one die for each class, and use the *lower* result as the die type. In other words, suppose you have a Fighter/Magic-User. Roll a d4 and a d10 (depending on which version of the game you're playing, of course). Whichever die is the lowest, you use that as the basis of h.p., although you may max it at first level if you choose. So if you rolled a 3 on the d4 and a 2 on the d10, you'd start out with 10 h.p. But with a roll of 3 on the d4 and, say, 6 on the d10, you start out with M-U hit points. The rationale behind this is, the less combat-oriented one of your classes is, the less likely your character would be to have learned the skill of mitigating damage.
In this way, players can eventually free themselves from the supposed restrictions that class-based gaming imposes, without too much power imbalance as a result.