I'm bad at being that customer, but I generally check the line behind me and make an effort to not be that guy.
My personal favorite though, is the guy or gal that bitches, out loud, the entire time they are in the line. "Can't this guy hurry up?" "What's taking so long?" "That lady isn't even doing anything!" Then as soon as it's their turn, they launch into a personal story that has nothing to do with the transaction they are there for. Are you freaking kidding me?