To avoid a gummy, overstarched mess, mash the potatoes while they're dry but still hot. Also never use a food processor or blender. They usually overmash the potatoes. However if for some reason yours became gluey, you can make a gratin instead by spreading the creamy mashed potatoes in a thin layer across a baking dish and topping with some butter, cheese and breadcrumbs. Bake to form a crispy top.