"Restaurants with table service: Tip 15% or more of the bill, based on the quality of service. If you receive exceptional service, 20-25% is customary. In major cities of the U.S. however, 20% is considered to be a "good tip."
http://www.tripadvisor.com/Travel-g191-s606/United-States:Tipping.And.Etiquette.html