I tip 15-20%, mostly because I appreciate my hairdresser who is very accomodating to my habit of scheduling last minute appointments ("holy cow I can see 4" of root and split ends! I need it fixed RIGHT NOW!" ;) ) and once redid a perfectly good haircut that I just didn't like and refused to let me pay her for her effort. She also gives me all kinds of samples of stuff.
So my tip is an appreciation of her going above and beyond for me.