Based solely on 3 of years as a waiter, part of that at a TGI Fridays, and part of that at a somewhat nicer establishment, here are the stereotypes/prejudices that I developed:
Black people were consistently very good tippers. I don't think I ever received a crappy tip from a black person, male or female.
1 woman alone tips poorly. 2-3 women tip well. 4 or more women = a really lousy tip.
Smokers tip better than non smokers.
People who drink whiskey are good tippers.
Anyone out on a date tends to tip well.
Europeans tend to tip poorly. Middle Easterners also tend to tip poorly.
College kids out drinking don't tip well.
Middle aged men are pretty consistent 15-20% tippers
White rednecks tip well.
And lastly, on a more introspective note:
Good service has a strong correlation to good tips. Generally, the better service I provided, the better the tips on average. There were exceptions to this, however, I definitely made better tips when I was on top of my game.