Good posture used to be taught to young women but now it seems like everyone is slouching. Perhaps it is a sign of the times because of our addiction to our smart phones - causing us to hunch forward. The problem is that a rounded thoracic spine (kyphosis) can lead to host of issues: shoulder dysfunction, neck pain, balance issues and even compression fractures.
To build the strength of the mid back muscles (those muscle between your shoulder blades - mid trapezius and rhomboids), it is necessary to do exercises where the shoulder blades move toward one another (adduction of the shoulder blades).
The video below depicts a shoulder blade retraction exercise done with the Hooked on Pilates HANDIBANDS. In the video the HANDIBANDS are affixed to a pilates cadillac table. However, they can be attached inside a door jamb to do the same exercises at home. Do 15 repetitions palms facing forward and then 15 repetitions palms facing back. Make sure to keep the ribcage stable and aligned over the pelvis as the arms works.