Don't say "excuse me" when what you really mean is "excuse YOU!"
I find it hilarious that we can me passive-aggressive with strangers in millisecond confrontations, but it's sadly true. The situation is always one where an "excuse me" is not called for: You're walking blithely down the street and someone briskly pushes past you and says "Excuse me!" even though there's plenty of room to walk around you without any physical or verbal contact. The message is "you're walking to slowly," not "I'm politely pushing past you."
People always say "excuse me" when they mean "excuse YOU!" in lines, like checkout lines, as a means of saying "I was here first!" so you don't cut them. There's usually no way you were going to cut anyway. The "excuse me" is always uttered in a distracted, neutral tone, as if the person were just refexively dropping a token nicety because they're polite, instead of fiercely staking their claim to one square foot of real estate in the universe.
"Excuse YOU!" is, of course, outright rude, but it gets the message across. People say this when they think you've just done something that required an "excuse me": They're pointing out what a heathen you are. A misused "excuse me" comes off as haughty, brusque -- it's a offense play against a clueless stranger.
Obviously, "excuse me" should be invoked when you truly want to excuse yourself. Come to think of it, I think I used the passive-aggressive form of "'excuse me' when I meant 'excuse YOU!'" the other day. I was in a long taxi line at LaGuardia, and the guy behind me kept creeping up so close that he'd trip over my luggage. After the fifth time, I smiled sweetly and said "Excuse me." (So bitchy! But I was tired! I was home! Forgive me, dear reader!) He, however, handled my maneuver quite deftly, "That's okay, ma'am" he said, taking me at face value instead of giving me what I was unconciously desiring -- an apology. He took the fangs right out of my passive-aggressiveness.
I've said it before: Good manners are a tool that can be used for good for evil. Having good manners does not give you the right or the responsibility to educate the ruffians in your midst. Good manners should be used to make other people feel more comfortable, to grease the wheels of society so we get along better, not to create a caste system in which haplessly rude people are taken to task by their finishing-school-tutored betters.