I personally take a stance similar to that of Mr. Pink from Reservoir Dogs but with the exception that I only rarely never tip. Generally speaking, bad service = no tip or change up to a dollar. Mediocre to alright service (meaning they at least fill my glass when its empty) will get an 5 - 8 percent tip max. Anything above and beyond that, they get anywhere between 10 to 20 percent.
That all is assuming the server doesn't provide sexual favors, which hasn't happened yet, but you never know... lol.