Just because they include gratuity on you're bill doesn't mean that you have to pay it.
If they include gratuity and it was poor service I simply deduct it and pay the amount that I feel was appropriate. If I really have a problem with the service I generally ask to speak to a manager.
Oh and if you really want to piss a waitress off flip an empty cup over and leave pennies on top of it, that way they know they fucked up.