Hitting behind the ball, commonly known as hitting it fat, is usually caused by changing your posture somewhere during your swing.

If you lift up on your backswing a few inches, then you have to come back down that same amount to contact the ball in the position that you were in at the address position.

By maintaining your posture throughout the swing, your shoulders will move properly and allow the clubhead to strike the ball in a descending manner.

If your right shoulder comes down and under before you make contact with the ball, you will hit the ground behind the ball. Make sure you keep your trailing shoulder up through the shot.

A good drill to learn this motion is to take a shorter club such as an 8-iron and get into the address position. Remove your left hand from the club and swing the club head back and forth with your right hand and just brush the ground. Left-handers will do this drill with their left hand.

To keep from hitting the ground behind the ball, you will have to keep your trailing shoulder up on the downswing. This drill will cure your fat shots and improve your posture during your swing.