To fix this problem, as you start your squat, reach back with your hips and simultaneously point your chest at the floor as you descend (watch the included video for a demonstration). In other words, get leaned over as you descend - it will put you in a stronger, more efficient position at the bottom of the squat so that you can effectively drive your hips up out of the hole. Trying to stay upright (and trying to get upright too soon on the ascent) simply makes your squat harder and less efficient than it should be.
Mistake #2: Not Shoving Your Knees Out
When you squat, your knees travel forward - after all, that’s what knees generally do when they bend, but you are also going to shove them sideways, i.e., you’re going to shove your femurs apart from one another. We don’t want your knees to travel only forward; instead, your knees are going to travel both forward and out (i.e., forward and sideways).