Please note what sunshine just said. Please remember that many servers make minimum wage. I didn't realize how low minimum wages are for restaurant workers until she posted that.
If you order an $8 entrée and a $1.75 Coke, when you tip 15%, the waiter only gets $1.58.
The waiters at more expensive restaurants do better. If they have a $30 entrée, $20 in drinks, they make $8.07 at 15%. But that wouldn't pay for a burger and a Coke at a cheaper restaurant.
So please remember to tip generously, especially at less expensive restaurants. The amount is really more important than the percentage. I mean, 15% of a small amount of money is virtually nothing. So crank it up to about $3 a person or about 25%. Yet 15% of a large amount of money is more, yet wouldn't pay for a burger and a Coke for the waiter. My rule is 20% and more if the service is great.
If the service is bad, is it the kitchen's fault or the waiter's fault? If you are unhappy, talk to the manager. Don't leave and just stiff the waiter. The manager should appreciate constructive criticism.
I like how George's does what Houston's does. They print 15%, 18%, 20% tips on the ticket so that you don't have to do math in your head after having a couple of drinks (or even worse, get out a calculator to compute a tip).