Well, here is a quick review: http://survivinginfidelity.com/faq_bs.asp#FAQ11
Vaguely friendly, but distant. Think being polite to someone your parents invited over, who you don't know very well and don't have anything in common with. Non-commential or feeding comments back. "Didn't MIL give DD the best gift ever? - What a nice gift (said with a vague smile)." Friendly, but distant. And make spaces for yourself to get away, when needed. Lie-down for a headache, etc.
Best of luck. Just keep thinking of the time when everyone goes home and you can take care of business. (((hugs)))