These excellent instructions on how to make the perfect mashed potatoes as well as this mouth-watering photo came from Our Best Bites.
1 Potato for each person (If you use Russets, it's probably best to peel them, or at least most of them--their skins tend to be tough and unpleasant in mashed potatoes. If you like skins, feel free to leave them, or most of them, on red or Yukon golds.)*
Milk (you could also use buttermilk, whipping cream or half and half)
OPTIONAL: add-ins such as roasted garlic, a clove or two of finely minced fresh garlic, smoked paprika, garlic salt, bacon, cheese (shredded sharp cheddar, blue cheese, shredded pepper jack...), chopped green onions, chopped chives
Bring a large pot of salted water to boil. While you're waiting for it to boil, chop potatoes into approximately 1/2" cubes. When you can easily cut a piece of potato with a fork, drain potatoes and place immediately in a large bowl, preferably of a heavy-duty mixer. Add about 1-2 tsp. of butter per fist-sized potato and mix on medium speed with the wire whisk attachment. If you're adding cheese and want the cheese to be melty, go ahead and add it now, otherwise add it when it is time for the add-ins.
Slowly add enough milk or buttermilk to achieve a nice, fluffy consistency. Turn speed to high and add desired seasonings. When the potatoes have reached the level of fluffiness you like, go ahead and add any additional add-ins while mixing on low. Adjust as necessary and serve immediately.
*The starch content is higher in Russet potatoes, so they build up into fluffy, light mashed potatoes whereas red and Yukon gold potatoes will be pretty creamy, but they can border on runny if you're not careful.