You don't have to say no. Just say "not TOO much." Drink a ton of water before the meal and remember what a serving size is. Eat slowly and if 20 mins later you still want more, have a LITTLE more.
I made thanksgiving food this past weekend just so I wouldn't find myself going overboard on Thanksgiving. I think many of us end up going overboard with Thanksgiving because we don't make that stuff for ourselves during the rest of the year.
I don't think I'm going to have a problem with Thanksgiving this year, but Christmas will be another story....tooo many goodies!!!