Another novice mistake is to point the elbows backwards during the squat. Not only is this bad for the shoulders, but it also creates a poor foundation for the bar to rest on and makes it easier for you to round out in the bottom of the squat. If you have difficulty pulling your elbows down, you can grip the bar with your first three fingers only, it takes some getting used to, but a lot of big squatters have improved their technique this way.