We've had this discussion on here before, but my baseline tip is about 10%. In other words, 10% (probably a little more since I round up to make things even out in $.25 increments) is what I give for average service. They have to earn it over that. But since I don't go places where I've been burned by bad service, it usually ends up being around 15%.
That's a bit low, but it's your money.