There are some good suggestions here (ignore the title it works for most social occasions)
Crying Out Now: Thanksgiving Survival Guide
I can;t stress enough the thinking ahead part.
If you're known as a drinker have a response really for when someone asks you if you want a drink or asks you why you're not drinking...
the reason doesn't have to be a long essay about your alcoholism with you standing on a table making a speech...something like 'I don't feel like it', 'for my health' I'm driving' are all valid reasons.
Have an escape plan too just in case it all gets too much- it's ok to be seen to be there...and then leave after a little while.
D