I can't stand it when waitresses/waiters aren't pleasant. You don't have to be super cheery and make jokes, etc., but nothing's more unappealing than being served by someone who's visibly pissed off. And then I hear her cursing as she goes back to the kitchen to get a bottle of ketchup. What did she have to do after she brought it back to the table? Nothing.
That's pretty much it, but I'm low maintenance. Just give me my food in a polite and timely fashion and you'll get 20% out of me, easy. Flirting, ass grabs and like gestures are also easy ways to squeeze an extra 3-4% too.