This is merely a public service. I am not talking about the family I just visited last weekend (uh, hi Dad). Family gatherings are often fun and pleasant. This post is just for those few people who have family they'd rather avoid.
1. If you have a job with vacation days, use them all up before the holidays and tell your family you have to work.
2. If you're flying, give up your seat when they ask for volunteers to accommodate stand-by passengers. Ask the stand-by person who's getting your seat to give their best sob story so you can tell your family how their need was greater than yours.
3. Start volunteering with an organization that's active on Thanksgiving, Easter, Christmas Day, etc. Make a commitment to help out on the holiday and then tell your family you can't possibly let down the children or the elderly or the botanical plants or whatever.
4. Start a serious, long-term relationship and divide your holiday time with your partner's family. Warning: this only works if their family is not as bad as yours.
5. Leave food out that is easily spoiled (mayonnaise, raw chicken, etc.). Eat it about 12 hours before you're supposed to leave. Call to cancel your visit when you're at the height of the food poisoning symptoms.
6. Tell those family members who you'd rather not see, exactly why you will not be visiting. This won't be an easy conversation, but it will be honest and might help them look at themselves in a new way. At worst, they'll stop talking to you and then you'll be all set for next year.
Good luck and Merry Christmas!