Any time you go to throw a ball without moving your upper back and thoracic spine, the shoulder is forced to pull double duty, enduring excess mechanical stress. Do this enough, and labral injuries and rotator cuff tendinitis start to develop. To prevent such injuries and reverse any imbalances you may already have, you need to mobilize the thoracic spine to ease the stress on the shoulder. Here's how it's done.
On a mat, place a foam roller on your thoracic spine--the portion of your spine between your neck and mid-back. To get your shoulder blades out of the way, cross your arms over your chest.
Tighten your tummy, and roll up and down four times.
Tip side to side four times. This loosens the muscles in your thoracic spine.
Take in a big breath, lean back over the foam roller, and exhale slowly. As you do this, be sure you're stable and the motion is coming from the thoracic spine, not the thoracolumbar junction.