Of the 4 main rotation centers in the golf swing, the shoulder is one that many golfers know they need more of, but never seem to improve the shoulder range of motion as intended. You primarily need shoulder external rotation in the trail shoulder to allow for a full backswing, which is measured in standing and in golf posture. If you raise your arm to the side of your body to 90 degrees and rotate the top of the hand up and back (external rotation) you should be able to at least get your forearm perpendicular to the ground. If you can’t achieve this position (past perpendicular is best) then give these two exercises a try.
To correct a shoulder external rotation limitation, you want to first stretch the shoulder into the position and then activate the muscles in the new range of motion. An easy stretch that can be performed with a golf club is completed by placing the head of the club in the hand and placing the shaft behind the forearm. You then grab the shaft of the club and use your elbow/upper arm as a leverage point and pull yourself into an externally rotated stretch position. Hold for 5-10 sec for 5 reps. Then you want to activate the muscles into the new range of motion, so grab a band and set up in golf posture. Tension the band, and rotate into your backswing while externally rotating the trail shoulder against the band resistance for 10 reps, holding for 3 seconds each rep.
These 2 simple drills will make a huge difference in your ability to load the backswing and allow you to stay in posture while doing so. Give these a try and start hitting bombs off the tee!