Keep Heels to the Floor When You Squat
When squatting, it is important to keep your body weight on your heels. If your heels rise off the floor then there is too much pressure on your knees. This is a problem, especially if you have a weight on your shoulders or back.
If staying flat-footed during the squat is difficult, it's best to perfect your form over time. Squat down only as far as you can go without lifting your heels; then, as bending beyond this point becomes easier, gradually bend down more. Your goal should not be a deep squat, but rather one that is effective and positions your thighs parallel to the floor.