I was taught by a powerifter that is # 1 in his weight class(he's 285lbs at 5'7") in the province how to squat. First, he showed me to sit the bar low on my back, say bottom of my traps, it's easier to keep the bar in a straight up and down motion during the rep and keep your centre of gravity from tipping forward, also, when coming out of the hole, drive with your hips, and if you feel yourself tipping forward, push you hips through like you want to screw something. When he showed me this method, my squats went from 3 plates (315) for reps, to over (405) within the first couple months. Incidently, last time I spotted him, he buried an 800lb squat like he was screwing around.