The shoulder turn is crucial in the golf swing and unless your shoulder turn is correct your golf swing will be compromised. During the swing be constantly aware of a smooth shoulder rotation around the upper spine. It is vital to avoid raising or lowering either shoulder. The shoulders rotate on an inclined plane and guide the the whole swing process in an essentially passive manner.
If you are rocking your shoulders (raising or lowering them) during your shoulder turn, the quality of your contact with the ball will be erratic at best. The chances of your club face arriving at the ball in the position you had it at address are small because one or both of your shoulders have moved relative to where they were at address. Try this simple drill to feel the “passiveness” in the shoulder turn. Adopt the address position (see the Address Position post) and fold your forearms across your chest with each hands fingertips on their opposite shoulder. Rotate the shoulders smoothly around the upper spine and develop the feeling of a smooth unitary turn.
If you retain a feeling of “passiveness” in the shoulder turn you will not go far wrong. Be aware of the shoulders throughout the swing and try and keep the shoulder muscles relaxed.
I find that by restricting my attention to the shoulder turn only during the swing and keeping the rest of my body and hands supple and relaxed I achieve very consistent results. Try it yourself, first with chips and pitches then work through to your long game.