Use heavy cream or sour cream instead of milk.
Use real butter not margarine or substitutes.
Use a hand masher or ricer.
After boiling and draining the potatoes, return them to the pot, and leave on the burner for about 30 to 60 seconds or until excess liquid evaporates from the potatoes. They should look "dry" and flaky, not slick, shiny or wet. Absorbing the extra water helps with getting rid of starchiness.
Only use a hand mixer if you are going to do twice baked.
For flavor versus a plain mashed potato, add mashed roasted garlic (put entire garlic bulb in foil and bake until softened about 15-20 minutes) sea salt, course ground pepper, finely diced chives, bacon, aged cheeses, are all nice when added to potatoes together or separately to taste. Always use fresh seasonings/herbs, not dried for the most flavorful punch. There is a difference in flavor when you use fresh garlic versus powdered etc.
Small red potatoes are great mashed with the skins left on. Larger potatoes like Gold Yukons for instance make great twice baked potatoes where you basically bake them, scoop out the insides, mash and season them, put it back in the shells and broil for 5 minutes to crispen the tops)
One of my absolute favorite versions of a type of "mashed" potato is to take several small red potatoes, and boil them til softened. Line a cookie sheet or jelly roll pan with foil, put the potatoes on the foil about an inch apart, and smashed them - skins and all- with a masher until someone split open and flattend. Then season with kosher salt, course ground pepper, rosemary, and olive oil, and broil for about 5 minutes or until crisped. Top with a pat of butter, bacon and bacon drippings if you're not watching your weight, a dollop of sour cream, and top with finely chopped fresh chives. They are very crunchy on the outside but very much like a decadent mashed potato on the inside. Smoke alert though! The olive oil can and does fly under the broiler, so you need to keep an eye on them so you don't have a fire. Be careful taking them out. The olive oil stays active for a good while...popping and snapping and all.