Mashed potatoes will thicken as they cool since potatoes are a starch. This is why I always make the potatoes right before it's time to eat. Something I like to do to help keep them from thickening, though, is to leave them in the pot with the heat on low and keep a lid over them. This will help them keep some moisture in the pot. If they get too cold, they will thicken. So make sure they remain warm. You can also add a splash of unflavored/unsweetened non-dairy milk or even a little chicken broth to make them creamy again.