I learned from my mother to always give a tip of at least $20, my mother once worked as a waitress back when she lived in Texas very long ago, may she rest in peace. I expanded on it and I start with a $20 tip and then add $5 dollars for ever additional adult that eats with me. I know it's a large tip but I just grew up doing it and I've only once received bad service, I tend to go to the same set of restaurants though :P