I always leave 20% unless the service was super crummy. I cut my own hair so no problem there. I do pamper myself and have acrylic nails and I give my lady $5 to $6 each time I have them done and that is just a little over 20% of the cost. I've been going to her for six years and so I also give her little gifts at Christmas and her birthday.
My niece waited tables while she went to school. I for one could not be a waitress because I'd screw up so bad I'd get fired in a half hour. A good waiter works hard and the money is really crummy so tips really count. When a person is kind and caring I figure they deserve to get as good a tip as I can afford.