For a dine-in restaurant, 20% is a good place to start, and I generally "normalize" somewhere between 25% and 30%. I've gone much higher, and much lower depending on the service - once I even left a "negative tip" when the (horrible) waiter was too lazy to make correct change, assuming that I'd just leave it all (he only lost 15c, but I hope my point was made).
One thing that drives me nuts is people who scale back the percentage when the bill gets high. You know - they don't mind leaving 15% on a $30 tab, but if the group bill is $200, they ain't leaving a $30 tip! Those people drive me nuts. I figure if I can't afford to leave a decent tip, then I can't afford to be eating out.
For delivery guys and takeout, I'll usually go 15-20% as the service is typically less - but if it has been excellent, they will be rewarded. One pizza guy included his own tip when he told me the price (it was different than what the bill he handed me said) so he got no tip, and a stern warning about ethics and the possible ramifications he might face from his manager if he ever tried that again.
I agree wholeheartedly with TomD and others who have said that you should always tip well and be extra courteous to places that you frequent regularly. I don't have a local watering hole any more, but I still recieve excellent service and a warm welcome when I go back to the pool hall/jazz lounge I used to hang out at.
<If you're gonna be dumb, you gotta be tough>
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