Place a broomstick on your back across the shoulders. Assume your normal golf posture - If you are unsure as to what a good posture looks like, refer to the posture tip article. At this position, place a strip of tape on a mirror at the same level as the top of your head.

Next, practice turning back with the broomstick, picturing that the shoulders are turning perpendicular to the spine and that the spine is remaining constant in the same angle throughout the swing. 

As you turn back and through the top of your head should remain on the tape in the mirror. Although the head can turn and move back a few inches along the strip of tape to assist in a weight shift, it must not move up or down. If it does go up or down, you have compromised one of the four levels discussed previously.