This answer depends upon the position you prefer to sleep in.
Sleeping on your side. If you sleep on your side, you should lie on your unaffected shoulder and place a thick pillow under the arm of your affected shoulder. This pillow helps to elevate your arm and lower pressure.
Sleeping on your back. If you sleep on your back, you should place a pillow under each arm. You could also consider sleeping with a wedge-type pillow that supports your upper body.