Well...In my family the normal thing to do during this kind of dinner is eat so much that you could roll away from the table afterwards.... What I do:
1) No seconds (the other ones always have seconds and some even thirds)
2) Fill up on veggies, without being greedy of course.
3) I don't eat a full plate
4) I don't take much gravy
5) I limit desserts to max 2 table spoons. If it's something I like. Not a desserts person really...
6) Even though there is chocolate on the table after dinner, doesn't mean I have to eat it.
With that being said, we don't celebrate thanksgiving or anything over here, so the only issue is christmas.
I hope it goes well for you, and I think stimkovs makes some great points!