I tip for haircuts and food the same way: 15-25% depending on how you did.
In a restaurant I'm pretty easy to please. Get my order right and refill my glass without me asking. That's it. I have a lot of friends in the food industry so I tend to tip more generously. I treated it like a Karma thing and in way it was. When it came up in discussion, my old girlfriends and dates all said that they appreciated that I left a generous tip and ordered drinks and appetizers. I guess it made them feel more comfortable about what they could order and didn't make me look cheap. I didn't even think of it that way, but good to know!
Haircuts I pretty much just tip everyone 20% unless it's ruined, which is rare. If the job is mediocre, I just won't come back to that person the next time.