Stand tall (think: tall spine) facing a mirror with your arms hanging directly by your side. Next slide your shoulder blades back together as far as you can, literally attempting to touch them together, while puffing your chest up. This should flex the muscles of your upper back (rhomboids, middle and lower trapezius), pulling your shoulder blades back and down. Now while holding this position raise your arms out in front of you, not changing anything about your shoulder blades. The shoulder blades should stay back and down, and definately should not shrug up.
What you're doing by raising your arms out in front is activating your anterior deltoids, inhibiting your posterior deltoids. I have seen countless athletes/people substitute scapular retraction and depression by simply externally rotating and/or extending the glenohumeral joint by using the posterior delts. This can potenially be a major problem that often leads to shoulder impairments.
Work on developing the proper motor pattern of scapular retraction and depression by practicing the above mentioned exercise. Focus on sliding the shoulder blades down and back when performing all upper body pulling exercises. Actually, all exercises, including lower body, will benefit from this. Don't forget to maintain a tall spine. By perfecting this pattern, you'll be less likely to have shoulder issues and you'll definately make greater gains.