Waist length here, and I just put it in a low bun just under my helmet. That way it doesn't flop around and tangle (or at least not any more than it would doing other day-to-day activities). I have to admit I've never gotten the hang of doing a single braid down my back, so if I'm going swimming mid-ride (I used to ride out to Walden Pond to swim a fair bit) I'd put it in two braids and then twist the braids into the same bun, so that if it came out of the bun while swimming it wouldn't tangle around my shoulders.

My real issue with my hair & riding a lot is that I have to wash it so often to get the sweat out -- it's really much happier only being washed every 4-6 days (which I promise is not gross if one's hair prefers it), but I a: have nasty sweaty hair and b: get dandruff from the nasty sweaty hair if I don't wash it after every ride long enough to get sweaty-head. Alas.