Here is a mobility exercise video that will help increase the shoulder turn in your golf swing. You want to ensure that the majority of the rotation in the golf swing is coming from your upper back. Often times golfers will over rotate through the low back which can lead to low back pain.
The shoulder turn is not only dependent upon the golf set up posture, but also the actual mobility of the upper back (thoracic spine) region. This exercise takes advantage of the natural biomechanics of the thoracic spine and the relationship of the movements rotation and side bend. These 2 movements occur in the spine together especially in the golf swing.
So give this a try and watch that shoulder turn improve.