Fellow Band-Members as Mentors.

I have often remarked that the ideal way for anyone to really learn to be a musician is to work with good musicians. OK, that seems a tautology of sorts, or perhaps just trivially true, but there's more to it than seems.
Much of performing (as opposed to studying) music can not be written down. Some composers think that what's on the printed parts represents about 40% of the actual performance. I suspect the percentage is even lower, especially if one considers the inaccuracy/vagueness of things like articulation and dynamics. (e.g. - how loud is an sfz, anyway?)
So the best, and I think only true way, to learn that remaining 60-70% is by a type of apprenticeship. Working with players who know more, or perhaps just do more, about the actual performing part of musicianship. It's more like osmosis than anything specific. And it happens a little at a time, in sometimes unconscious/subconscious ways.
For instance, I remember quite vividly the time I discovered what it feels like to have a band "swing" or "groove". This concept was always a mystery to me, being classically trained. And I had been working as a pro musician for several years, not really "getting it". Happened one evening in the middle of a tune, nothing special going on, but the drummer had that groove happening, and suddenly things fell into place. Ho! Indescribable - literally. (Thanks Nick Adams, wherever you may be now.)
It's a Zen thing - you know when it happens, but exactly what it is, nobody knows. Miles Davis called it that "special thing". And he got right. So how did I learn it? By osmosis? Dunno. Nothing ever really was said. Just played. But now I know.
A little story to end this meandering:
One evening at the T.H., we had a young sax player subbing - RM, let's say. Still in high school, even. Tons of talent, but inexperienced. The piano player, on the other hand - KC let's call him - had been in the biz quite a while, probably 15-20 years. And he didn't cut anybody any slack. (If you're on the stage, you should pull your weight - otherwise go home.)
The first set was hell for the sax player. KC didn't give him any room - unexpected inversions, ambiguous voice-leadings, you name it. And of course RM was mostly lost, especially playing tunes he didn't know very well.
So the end of the first set, RM went to KC and the whole conversation was as follows:
RM: "Give me a break, I'm only a kid."
KC: "NO."
So at the end of the second set, the conversation continued:
RM: "In five years I'm going to be lot better."
KC: "Me too."
Well, it's been a lot more than five years, and it's all been true.