Opening these chest muscles will allow for better posture, as they are common culprits in holding the shoulders forward, causing the overstretching of the rhomboids between the shoulder blades, which is where the poor-posture-pain is usually felt.
Using a foam roller of about 6” in diameter and 30” long, place the foam roller (or a long narrow cushion, or even tightly rolled blanket) on the floor and lay on it, with your spine contacting and resting upon the length of the roller. Your head should not drop off the end of the roller, but also rest upon it comfortably. Put your arms out perpendicular to the roller (and spine) and let them rest, falling aside to the floor. You will feel a nice stretch in the pecs, opening them up gently. Hold as long as you can, a minimum of a few minutes, breathing deeply. During the first minute of the stretch, the elastin fibers of your muscle tissue will release, but the longer you hold the posture, as with any yoga move, the more you will get the collagen fibers (the more dense connective tissue that makes up tendons, ligaments and fascia) to lengthen; which is said to be attained in about 3 minutes.