You need to perform a lot of stretches for your chest and shoulder girdle. Then you need to strengthen up the muscle of the mid and upper back. By losening up the front of your body and strengthening up your back your body will begin to pull itself upright. You need to strenghthen up your mid and upper traps, rhombiods, levator scalpulae. These are some of the main muscles that govern the posture of your back.