Spilled wine on the carpet? Surreptitiously move an area rug from another part of the room so it covers the stain. If there are no rugs handy, use a sluggish pet (e.g., fat cat) or another guest's sweater.
It's always a mistake to offer alcohol to an underage relative; he or she will think you're a dweeb. You can't be expected to know the drug du jour, so simply point out the medicine cabinet and look the other way.
Wear socks with plenty of holes in them in case your tightly wound hosts make everybody take off their shoes before walking on the rug. Serves 'em right.
At a family style dinner, all serving plates are passed to the right. One exception: if somebody to your left asks for seconds, you don't have to pass the plate all the way around the table again (duh!). Just give them a cold stare and ask "Haven't you had enough?"
Your attire should be appropriate for the home you're visiting. In some cases this means it must be spit-up-proof and pet-hair-repellent. It's also tacky to show lots of thigh or cleavage if most of the guests are over 50, because you might induce a heart attack or stroke. When in doubt, opt for the little black dress (unless you're a guy).
If a hostess gift is in order, consider a fancy picture frame with your photo in it, a spray canister of Endust, or a McDonald's gift certificate.
Concerned about keeping up your end of the conversation? Remember, most people like to talk about themselves. The trick is getting them to stop.