There are a few different ways you can keep food warm at a potluck. A crockpot is a great way to keep dishes warm, since it has a 'Keep Warm' setting that will keep your dish at a warm temperature without overcooking it. You could also keep the food in a dish covered with aluminum foil, which should trap the heat inside and keep the food fairly warm. Chafing dishes are also a great option, although they're not something most people have on hand!