T-3 Days: Don't be that guest.

I know I harp on this every year. But seriously people! Don't show up empty handed to Thanksgiving! Don't show up at 5 when dinner starts and leave at 7 before cleanup. If you are the hostess, do not be shy about drafting your guests into cleanup duty! If they are close enough to you that you invited them over for Thanksgiving dinner, they are close enough that you can ask them to do the dishes. If nothing else, when you play the thankful game, toss out a, "I'm thankful that you guys are going to do the dishes."
My Uncle John always shows up with a pie, and his wife always helps clean up. One son brings something and the other carves the turkey. They are on the list of "good guests". The list of bad guests? One set of family members that shows up at the start of dinner and then leaves before it is over, avoiding bringing anything or set-up/clean-up. On the list? The husbands who let their wives do all the work and then watch football. The wives that let their husbands do all the work and then watch football.
There is a concern about "too many cooks" if you are sharing a kitchen with houseguests. Do what we do - cook in shifts. Plan a schedule. The night before Thanskgiving, I can tell you when the turkey goes in and my dad gets the kitchen, when I get the table for prep, and when I get the stove, and how many burners each of us get and for how long. I know which dishes go on the bottom rack of the oven below the turkey. I know what I have to cook in the crockpot or the toaster oven.
There is an issue when people bounce from Thanksgiving to Thanksgiving - it can be tough to keep dishes hot between one house and another. The most ambitious of us do it in shifts - prep two dishes. Bring one to Thanksgiving #1 and put the other in the oven when you get there - usually, the chef isn't using the oven once dinner is on the table. The better option is to bring a cold dish - desserts are best for this. This is my first Year of Two Thanksgivings. I'll be bringing pies to my grandmothers, which I plan to bake tomorrow night. (And then not eat. We'll see.) I'll also prep stuffing for my parents to bake and take over with the turkey. If you really can't bring anything, and really are so pressed for time, make sure you thank the hostess doubly and try to make up for it, either at Christmas dinner, by hosting next year, or just having them over for dinner sometime.
There are a million ways to help out at the holidays. If your mother/Aunt Sue/Uncle Charley/Grandpa is so controlling that they will not let you show up to Thanksgiving dinner with a side dish or dessert in hand, call them and offer to come a half an hour early to help out. Maybe you can take their kids for a walk or out to play in the yard so they aren't in the way. If you don't cook, offer to bring wine or sparkling cider. Sometimes, its really just the thought that counts. Calling and offering something can be a really nice thing to do. Or just let them know that they can call you and ask you to get ice/drinks/crudites/whipped cream from the grocery store if they forgot it. (A great option if you are traveling and can't cook.) If nothing else, make sure you stay later and clean up. Or when she/he gets up to clear the table after dinner, say, "Oh no, Aunt/Uncle/Grandma/pa/Mom/Dad - you did so much cooking for this fantastic meal that you do not have to clean up. Cousin/wife/husband/parent and I will take care of that." This way, you not only get to help out, you draft your lazy-good-for-nothing family into it as well. Just don't break the fancy china.
Nobody like a guest who shows up with one arm longer than the other. Don't be that guest.