When making mashed potatoes, heat the milk and butter together, over medium heat, until thoroughly warmed and the butter is melted (there should also be a few bubbles forming on the surface of the mixture). THEN add the milk/butter mixture to the hot, cooked potatoes. Apparently it is supposed to be way better than dumping in the cold butter and milk.
Also, mash with a fork if you have the patience. Supposed to give them a great texture!