I generally leave 15-20%, and always at least a $5 regardless of the bill total. My wife and I have a few local restaurants that we go to frequently, and they always take very good care of us.

I can only ever remember giving the $1 treatment once, and even that was too much. It was an out-of-town restaurant. The whole place was filthy, service was pretty much non-existant, plus I got a nice little case of mild food poisoning afterwards. Only time I've ever had that happen in my life