Lump-free mashed potatoes are a sure thing if you take the trouble to use a ricer or food mill, but you can also get them really smooth with an old-fashioned potato masher. The key is using the right kind of potato with the right technique, according to the talk on Chowhound.
First, choose starchy, lower-moisture potatoes like russets or Yukon Golds over waxy white or red potatoes, which turn gluey when you mash them. Once the potatoes are cooked, it's essential to remove any residual water before mashing, Violatp says. Drain well, then return them to the pot over low heat, shaking the pot till the potatoes are completely dry and look a bit floury. (Another option: Skip boiling, and steam or bake the potatoes instead.)
Without adding anything to the pot, start mashing. When the potatoes have broken down but still look chunky, add a generous chunk of butter and incorporate it thoroughly before adding liquid, GretchenS advises. Make sure to heat the milk or cream before adding. Then ditch the masher and use a sturdy wooden spoon to incorporate the liquid, mixing until the mash is smooth.
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Photo of CHOW's Sour Cream Mashed Potatoes by Chris Rochelle / CHOW.com