The chest muscles become very short and tight if you are repeatedly sitting in a slouched posture. The small muscle called the pectoralis minor muscle is the muscle most commonly shortened in prolonged slouched posture.
Stretching this muscle is an important aspect of improving posture. Because this muscle is attached to your ribs and part of your shoulder blade, when it becomes shortened it anchors your shoulder in a forward position and locks it down.
To stretch this muscle place your arm, forearm and hand flat on a wall or door jam, with the elbow slightly above the level of shoulder height. Rotate your body away from your shoulder until you feel a mild discomfort. Hold the stretch for 30 seconds and repeat 3 times. Perform this stretch sequence a minimum of 3 times per day to increase the improve flexibility. Depending on your job you many need to add this stretch as part of your daily routine to ensure continued good posture.