my divisions/categories:
UIL concert by grade level,
Christmas,
pop concert (movies, radio, etc...)
other concert (overtures, compositions)
Marching

I purge every summer: I know Acky Breaky Heart was a classic, but I don't think my band will ever play it again.

I put mine in alphabetical. I know, I know, but when I'm looking I look at the score and I find the score in the library. Yes, when I add, I get to move folders, but when I purge, I move folders also.