15% is standard for 'good enough' service. 10% is the min unless the waiter was beyond terrible or ruined your meal. 20%+ is for better than average on up.
I always try to over tip for the most part, because I know that so many people during a day will under tip, and you need those 1-2 people to come through and make up the difference.
It is really bad at lunch, where you are working almost twice as hard for about a 1/3 to 1/2 what you can make at dinner. Nothing worse than two ladies coming in (not to be sexist, but men are just better tippers), ordering a split appetizer and water, separate checks, and leaving you a buck on a $13 bill.