How do you make lump-free mashed potatoes?

Every time I make mashed potatoes, they get lumpy. I boil them for what seems like hours. They are well done. I just started keeping them in the fridge (but never made mashed potatoes out of them) if that matters at all. I cut them up into decent small sized chunks before I boil them. If you could direct me in what ingredients to add, how much, and how to do it, it would be great. I need a recipe for about 6 servings.