The number one lesson in our list is also the most overlooked. Beginners aren't used to being in a squatting position all day, so a lot of the time they'll straighten their legs, messing up their form, balance, agility, and control. Bending your knees does a number of things; it forces you to shove your shins into the front of the boot, gaining control of your ski. It also centers your upper body above your legs, keeping your balance over your sweet spot. When you bend your knees you'll also be more prepared to absorb small jumps and uneven terrain. Having straight legs while skiing is like having straight legs when trying to stand up on a bus -- either way you're going for a ride, it's just a matter if you want to be upright for it or not. Think you bend your knees enough already? Bend them some more and see how much better control you have through the bumps or down the steeps.
Editor's Hint: You're heel should always remain in the heel cup in you're boot. If it isn't, then you're probably not bending you're knees enough.