improving thoracic extension

One way to improve your posture and increase the range of motion in your shoulders is to open up your thoracic spine. While there is not a lot of movement available in that region, doing some simple exercises can stretch out your pectorals and latissimus musculature in order to make performing overhead activities easier. These are especially helpful for throwers and those that perform Olympic lifts.
 Perform 1-2 sets of the following for 10 reps.

Dowel overhead press
Place a dowel across your shoulders and pull your shoulder blades down and back. While holding that posture press the dowel straight up overhead continuing to squeeze your shoulder blades then return to the start position.
Dowel twists 
Place a dowel across your shoulders and pull your shoulder blades down and back. While holding that position try and rotate your mid spine to the right. Then try and move it to the left. This is not a large motion so if you are moving a lot it is probably from your lower back. Reset and try it again. 
If you feel cramping in the muscles around your shoulder blades you are doing it correctly. 
Immediately after completing these you should notice an increase in available motion at your shoulders.