To correct for this there needs to be a focus on the posterior delts. Rows, external rotation, and scupular retraction should be a basis of correction. Stretching of the pecs (both major and minor) needs to be included as well. A correction of this may take some time so be patient. Stop worrying so much about trying to have the best chest when the rest of your body is wack. In addition to correcting and creating a healthier shoulder joint you will also make yourself look better. Pulling the shoulders back will actually make you look bigger and have more of an "athletic" frame. Or for the females our there, pulling the shoulders back will make your two friends "sit up" nicely.
There are many aspects of fitness and training that most people ignore. Everyone thinks that chest and "abs" are the only things needed to be trained because that's what makes them look good. WRONG! Take care of your entire body. Work both sides, the small muscles and the joints, and you will see that you look better, perform better, and stay healthier a lot longer.