Also, people forget that the waitstaff have to tip out the bartenders and bussers in most places, too; it's not like your server gets the entire tip you leave.
Honestly, tipping is archaic and stupid. Just pay people a reasonable wage! IME the possibility of a tip makes no difference in the service I get at most places. Wait staff either take pride in their work and do a good job, or they don't care and it shows- and most people feel pressured into tipping at LEAST 15%, even when the service doesn't warrant a penny!