Constant head, chest, and torso position! [When I] pick my chest up out of the bottom, this shifts my balance. The barbell is no longer over the middle of my foot and it takes the tension off my hips, making it difficult to grind through heavy reps. Maintaining the correct torso position and knee position will allow you to stay in your hips during the entire squat... Now I'm looking down and out in front. I'm burning a hole in the ground with my eyes... Keeping my nipples down towards the floor helps me maintain the correct chest and torso position.