Ah well - things happen differently for me depending on which family we are having the do with.
With my family, we all bring heaps of food so there is
always leftovers - and it is all left with the poor sucker who opened their house to over 40 people, mostly kids!!!
With DHs family, there is meticulous planning. Who is bringing what, what gets cooked when and how, and then when its over, you take home whatever is left of what you brought.
I get out of worrying about it by always making stuff that there is never, ever, ever leftovers. Devilled eggs, rum balls, mousse. I also buy disposable or foil platters and crap to put it on so I can leave empty handed without contributing to the washing up.