[header] How to align your shoulders [title] Strengthen your mid back. [step] Part of being able to pull back your shoulders and keep them properly aligned is having strong muscles in your mid back region between your shoulder blades. The main muscles groups there are called the paraspinals, rhomboids, trapezius and infraspinatus. 
When these muscles are too weak, they allow the shoulders to slouch forward. When they're strong, it's easier to hold and maintain good posture.