Use Your Head About Shoulders
Fortunately, yoga provides a wonderful stretch for the rhomboids, the arm position of Garudasana (Eagle Pose). Cross your elbows in front of your chest, stand tall, and see if you feel a stretch between your shoulder blades. If you don't feel a stretch, try raising your elbows to shoulder height and also intertwining your hands and forearms so that your thumbs point toward your face. Whichever arm position you use, keep your breastbone lifted and breathe into the space between the shoulder blades. Let the inhalation expand the tight muscles, including the rhomboids; on the exhalation, feel as though the tightness drains out. Hold the stretch for one to two minutes and continue to breathe smoothly and evenly. When you are finished with this side, repeat with the other elbow on top.
Stretching Your Limits
Now that you've stretched the rhomboids, let's work on the lats and pecs. Roll up a blanket, small rug, or big towel to make a firm, round bolster. The bigger the roll, the bigger the stretch, so start small if you have tighter shoulders. Lie on the roll face up, with the roll across your upper back: It should be under the bottom part of your shoulder blades, not under your lower ribs.
Now stretch your arms up to the ceiling and feel the shoulder blades broaden away from the spine. Make sure the palms face each other, so you are incorporating external rotation, and stretch your arms overhead. Keep lengthening the arms out of the shoulders and don't let the elbows bow out to the sides. Breathe into the sides of the rib cage and visualize the lats and pecs lengthening with each exhalation.
While stretching the lats and pecs, it's best not to force the stretch to the point of pain. Pain signals that damage is occurring, and the nervous system tells the muscles to contract to protect themselves from tearing. Obviously, a guarded, contracted muscle isn't going to stretch very effectively. Also, it's generally a bad idea to create pain near or in a joint while stretching. The functions of the soft tissues nearest the joint, primarily tendons and ligaments, are to stabilize and protect the joint from abnormal movement. You don't want to risk overstretching, destabilizing, and damaging the joint, so if you feel pain while stretching your shoulders, support your hands on a block or on the rungs of a chair at just the right height so you feel stretch and not pain.
You may also want to bend your knees or even place your feet on the wall a few feet above the floor. If you have tight shoulders and stretch your arms overhead, your lower ribs will tip forward and your lower back will overarch. (This same mechanism can contribute to low back compression and pain in Warrior I and in Wheel Pose.) Both bending the knees and placing the feet up the wall anchor the pelvis and protect your low back from overarching.
Building Your Strength
If you work regularly on stretching back over a bolster, holding the stretch for at least two to three minutes, your shoulders will gradually open. To work in the new range of motion you've opened up, you also need to strengthen the muscles that flex the shoulders, primarily the deltoids, the shield-shaped muscles that cover the shoulder joint. One way to do this is by standing near a wall with your buttocks and shoulders lightly touching it. Slide one hand between your low back and the wall: A normal low back curve will provide just enough room for your hand.