For me, how much depends on the quality of the massage, just like the quality of the service in a restaurant. I would always tip at least $10, but have gone as high as $25-$30. I have a friend who is a massage therapist, in business for herself, not at a spa. She reports that about half her clients tip, and it is anywhere from $10-$50 depending on length of massage and any additional services ie deep tissue, hot stone, salt scrub etc. the percentage who tip might be higher at a spa. If I get a therapist who doesn't do what I ask (problem areas, deeper pressure etc), the tip is going to be the minimum, if the massage is extraordinary, so is the tip.