At my salon when you check out they offer you little envelopes for all the people that serviced you. I tip the shampoo person $7 (but they give you a head massage, not just a shampoo, yum!), the waxer gets $5 if I only have brows done, $20 if I have brows and bikini done (imagine how gross that job must be!!!). The hairdresser gets 20% of the cost of the cut and color, assuming I like the job they did, which I usually do. Once I had someone turn my hair streaky blond even though I described what I wanted done, which was NOT blond. That really made me mad so I did not tip them at all. I was especially mad because she knew she messed up--basically left the bleaching in too long, and then tried to convince me that it was a "terrific summer look." Bleech.
The nice thing about getting the envelopes at checkout is that you don't have to worry about how much you should tip at the time, since different people charge different amounts for their services, based on experience level.