Get warmed up
Before attempting to stretch at all, the first step is to ensure that your muscles are warm. You can take a warm shower, or perform any cardiovascular exercise. If you have just ended a working out session, you are good to go. Also ensure that your shoulders aren't injured, as it could cause you so much pain while you stretch, unless it's a mild sprain.
Stretch your chest muscles
After warming up, you can stretch. The muscles of the chest are really tight and they help to pull the shoulders forward. Stand close to a doorway and lift one arm, while you hold it straight and parallel to the floor. Now, grip the frame of the door, tightly, and lean forward subtly. Doing this will extend your arm behind your shoulder; causing a stretch in the muscles of your chest, upper arm, and anterior deltoid muscles. Maintain this pose for 30seconds, before switching to the other arm. Then, repeat for about five times.
Stretch your trapezius muscles
The next step is stretch your trapezius muscles; the upper trapezius connects the top of the neck to the shoulder blade and tip of the shoulder. It helps in the movement of the shoulders and is mainly involved when you shrug your shoulders. Try to stress one of your shoulders by stretching it so hard till the point of getting stiff; hold for 15seconds or more, and do same to the other side.
Stretch your rhomboid muscles
Rhomboid muscles make up the upper portion of the back and links the shoulder blades to the thoracic spine. They also help to retract the shoulder blades.
To stretch these muscles, find a pole, and grab it. Relax your shoulders as you pull back from the pole, while you extend your arms to feel the stretch between your shoulder blades. Remain in this position for 30seconds, and repeat.
After stretching your chest muscles, trapezius, and rhomboids, and you want to stretch more, the Back Scratcher Towel scratch is a good option, although a little complicated.
Grab a small towel; bend one arm over your head, while you travel the towel down your back. Now, bend the other arm around and towards the upper part of your back, to grab the loose ends of the towel. Move the towel to and fro down your back, and feel your muscles stretch. This part requires some flexibility, and not everyone can do it. Keep repeating and switching sides.