When I first got to the airport here, and had my first meal at Chili's (a popular barbecue food chain here) and was given the receipt, I was very unsure of how much to tip the waitress. I knew from the opening scene from "Pulp Fiction" that waitresses here run their butts off for their tables and typically make around 3 dollars an hour, which equals nothing to their paycheck after taxes. My meal was 7,50$, and I simply gave her a 10, and said that she could keep the rest(25%).
Now that I have been here for a while I have learned that it is common to double the tax or to give 15%. This goes for workers such as: waitresses, hairdressers, delivery people, cleaning personel and taxi drivers. This is to show that you appreciate their service and to make sure that you get treated good if you are a customer in the future.
In small towns it is common to give around 10%, but in major metropolitan areas like NYC or LA 20% and up is common to tip the waiters. If the service is good, if you are in a large group or if it is a nice restaurant you usually tip more. I guess the golden rule here is: If they take good care of you, then you should take good care of them.