Almost all the replies give you great advice for making good mashed potatoes, but few of them go to the question you actually asked. Dillbert's technique is pure Pierre Franey and so good he practically implied the answer. (Bravo!) But indianwells came closest.
Perhaps one of the problems was that your language is a little inexact and people didn't really understand what you were asking; but more likely it's just the general tendency to describe what one likes best. So much for psychology of posting. Let's get down to brass tacks.
The opposite of "stiff" isn't "light," it's "soupy." If you want stiffed mashed potatoes, use very little milk -- it's as simple as that.
The advice to dry your potatoes is good. Do follow it. The advice to use whole potatoes is also good -- you might want to try it. In any case, the less water you have in your potatoes, the better the milk will incorporate with the starch and the lighter your potatoes will be. Very little to do with "stiffness."
The advice to bake your potatoes rather than boil them is good also. It will net you a different, sweeter taste. It will also net a drier potato -- so lighter, fluffier mashed.
Different potatoes have different mashing characteristics. Dillbert was about as right as anyone can get with the Russet recommendation -- if light, fluffy mash is what's desired. Dill both spent significant time in Northern Europe and obviously knows his way around spuds. Coincidence? I think not.
Different mashing tools also yield different results. Ricers and mills will give you the lightest, fluffiest potatoesl, smoothest. Then mashers. Smashers (like pestles), the densest and lumpiest. Most people prefer mashers for "home style" mashed.
One thing that's death on potatoes -- no matter what consistency is desired -- is overworking. Try to mess with them as little as possible. If you want super smooth potatoes, rather than "whipping" them in a blender of mashing the heck out of them -- do invest in a ricer or mill. Or else just force them through a sieve.
No matter how little milk you use, make sure it's hot before mashing the potatoes. The butter should be soft, even partially melted -- but not completely melted for best incorporation.
Ideally, some of the potatoes will disintegrate as you first mash and the resulting paste will mix with the milk to form a "mortar." The mortar will hold the rest of the (still partly lumpy) potatoes together. For an extra stiff mash, don't use milk at all.
So, here's the basic "how to" recipe: Cook your potatoes thoroughly. Barely hodling together is fine (especially for Yukon Gold which go from undercooked to well done without much transition). Drain them thoroughly in a colander and return them to the dry cooking pan, over a low flame. Let them steam off for a few minutes then add your milk. That's just enough milk to keep the bottom of the pan from scorching. When the milk is warm, add the butter. When the butter is soft and starting to melt, add some salt and a few grinds of pepper (and perhaps some nutmeg). Now mash to the desired consistency. Adjust the seasoning. Yasou!
Hope this helps,