Usually 20% or so (move the decimal place to the left, times it by two). What's fun is watching a group of math majors figure out a tip ... they haven't used kitchen math in such a long time, they don't remember how to do it.
I also add more to it if we stayed for more than an hour or we were particularly demanding/loud and the wait staff was put under much duress.
My ex-husband was an awful tipper (pocket change type). Every place hated when he came in by himself. Waitstaff would tell me to sit in their sections when I was there.
Master brought up something interesting between our cultures ... In Britain, tipping is not practiced much (if at all), and this reflects in the poor serving attitudes in his country. He's always happy to tip whenever he visits America because the service is so much better.