Here's a link that should be bookmarked permanently on everyone's holiday browser: Cutting Through Holiday Jams With Etiquette as Your G.P.S. All of you poor, poor souls* hosting this year's Thanksgiving extravaganza should read it very carefully and perhaps "accidentally" send it to all of your guests.
Here's my favorite rule: Guests should always ask to bring a dish, but respect the host when he or she declines the offer. If you are assigned a dish you don’t want to make, too bad. Negotiating with someone who just invited you to dinner is not cool.
Although this one is a close second: If your brother’s ex-wife’s oldest son’s new girlfriend is making you feel edgy, or if someone is mouthing off about politics, it is acceptable to excuse yourself politely and take a little walk or see if you can help in the kitchen. Yelling, “I hate you!” and running from the table crying is not appropriate.
You can find more helpful tips here (although you'll just have to make up your own mind about a couple of inconsistencies between the two lists): CHOW'S 10 Party Preparation Tips.
I would also add that Xanax is always an appropriate hostess gift. That tip doesn't appear on either list.
*Hurray! Not me! Not me!