There are a lot of good points here from other chefs.
But, there are several ways of doing it.
Russets have a high starch content, and work well for mashed potatoes, but you can make good mashed potatoes out of any type of potato.
Some of the best mashed potatoes I have ever had were from Red Bliss, which do not have near the starch levels of other types.
A few tips that I find helpful.....
Get rid of excessive liquid, but do not cool the potatoes (exception described later)
Mash while they are still real hot.
Do not overmash. The more you play with them the better chance that you will turn them to glue.
Using a hand Potato Masher. Up and Down motions to break them down. Add the hot milk, butter, etc....and fold the ingredients in with a rubber spatula, wooden spoon, etc.....
When the ingredients are folded in........leave them alone.
Eat right away, or tranfer to a holding container, and keep warm.
(If you are fast you can add the milk, or cream, and butter without heating them--only softening the butter)
If you want to make small quantities at a time you can.....
Take Red Bliss, or Yukon Gold type potatoes.....Pre-cook, until cooked but still slightly firm. Cool, hold under refridgeration.
When ready to make the Mashed Potatoes......Place in a saute pan with a small amount of water....cover and heat until all the liquid is dissolved and the potatoes are dry, and hot (be careful not to burn).....Mash, add your other ingredients, and fold.
If you do it right you will not be able to tell the difference between freshly cooked potatoes, and the ones cooled completely.
If you make a mistake by adding too much liquid, and your potatoes become watery......... and you do not have any dried potato starch to add to them.......return to the heat, and fold in some bread crumbs......sounds weird, but you can tighten the potatoes, and make more of a "Country Style" Variety.
Sometimes I will make them this way on purpose because I like the change.
What do you do if your potatoes turn to glue?
Throw them away, and start over....
A Fresh Endeavor