Two key sets of muscles create and stabilize the ideal Sarvangasana, and these muscles are the ones that you'll need to strengthen. The first set, the shoulder's external rotators, includes the teres minor and the infraspinatus as well as the posterior part of the deltoid. The teres and the infraspinatus originate on the back of the scapula and run across the back of the shoulder to insert at the outer upper humerus (upper arm bone). Their fibers run primarily horizontally, so they have excellent leverage to externally rotate the shoulder. The posterior deltoid, the shield-shaped muscle that forms a cap over the shoulder joint, is the only shoulder extensor that has power and leverage when the shoulder is in 90 degrees of extension. The second set of stabilizing muscles is the rhomboids, which pull the inner border and bottom tip of the shoulder blades toward the spine (downward rotation of the scapula) and help hold them against the rib cage.
So let's begin working these muscles with an activity that provides light resistance and trains them to coordinate as you need them to. Stand with your back against a wall, heels a foot away from it, and knees bent a little. Keep your hips, shoulders, and head touching the wall, and your lower back on or near it. Notice again: If your palms face the wall (internal shoulder rotation), your chest is probably collapsed. Now turn your palms forward and roll your shoulders toward the wall and down, away from your ears. Doing all these things, and even pressing your thumbs to the wall, should put almost the whole back of the scapula against the wall, so that it supports the chest and prevents collapse. To strengthen this muscle group isometrically, press the back of your arms and hands into the wall, and keep your lower back and hips from lifting away from it. Working isometrically strengthens the muscles but does not change their length; this is the kind of strength that helps hold body parts in place in yoga poses.
The muscles you used to push your arms back into the wall are the same ones that push into the floor to lift you up in Shoulderstand. Setu Bandha Sarvangasana (Bridge Pose) is an intermediate step between standing at the wall and full Shoulderstand. You can use it to help coordinate this group of muscles and strengthen them more before putting the full weight of your body on them.
To do this, first lie on your back, knees bent and feet flat, arms by your sides with your palms facing up. Externally rotate your arms, pressing the thumb side of the hands into the floor so the outer shoulders press down and the chest opens. Keep pressing your arms down while you roll your spine and back ribs up off the floor. Repeat this action a few times; gradually increase the length of time you hold the pose to 20 or 30 seconds.!--page-->