Mashed potatoes do get stiff and gluey once they sit for a while. And while, to be honest, we'll eat them cold and hard, straight from the refrigerator, obviously you want them to be fluffy and creamy on your Thanksgiving table. But they cool down fast, and making them at the last minute isn't always an option.
One tip that we've used before (art also mentioned it) is to set your bowl of potatoes over a pot of simmering water, just like a double boiler. Cover the top of the bowl with plastic wrap or (our choice) a dish towel. Give it a stir every 15 minutes or so to keep the mixture well heated.
If you only need to keep the potatoes warm for about 20 minutes, we've simply covered the bowl with a dish towel and let it sit. We usually make our mashed potatoes in our standing mixer, so we just throw a towel over the top of the mixing bowl and the potatoes stay plenty warm for up to half an hour.
We've also made mashed potatoes the day before Thanksgiving and re-warmed them with great success. Here's our best tip for going that route: Add a healthy extra dose of cream and butter, so that the potatoes are almost soupy. You want them to just about drip off the end of a spoon. Put them in a deep casserole or soufflé dish—even a dutch oven will do—and store in the fridge overnight. Then warm them up in the oven the next day, uncovered, with a little butter on top. The extra moisture will evaporate and you'll have creamy potatoes. If you're really nervous about the outcome, do a test run to see how much extra liquid you need to add to get the consistency you want.
Any other tips?