How To Stretch: Extend Your Middle Back
Watch Dr. Greg Sher demonstrate some excellent exercises to improve your mid back flexibility and posture.
The big tip:
As with any exercise program, a slow and steady progression is very important to achieve an optimal outcome. A slow and steady build-up of flexibility allows your body to accommodate to the new position, and will allow your ligaments to stretch and your muscles to help hold you in better postures. You are better off doing this for a few minutes every day, than for a longer session only once or twice week.