Your arms are along your sides with the backs of the arms and shoulders pressing slightly into the mat to open the chest.
Inhale: Bring your knees up, deepening the crease at the hips.
Exhale: Engage your abdominal muscles and press into the floor with the backs of your arms and palms to extend your legs and bring them over your head, lifting your hips so that your legs are about 45 degrees off the floor behind you. Weight is on your shoulders and arms, not on your neck.
Bring your hands to the back of the hips to help support the spine.
Lengthen your neck - head away from shoulders, shoulder blades moving down the back.
Continue reaching your lower body upward, extending the hips and legs toward the ceiling. Do not sink into the shoulders and chest.
Hold five or more breaths.
Bring your legs back down over your face before you exhale to roll your spine down.
Also see Sarvangasana at About.com Yoga.
Pilates notes: What the shoulder-stand gives us is a supported way to work with the inverted position in some of our Pilates exercises like jack knife and control balance. In Pilates we usually move through this shape with arms alongside on the mat. Here, support from the hands can help you find the right place across your shoulders, not your neck, to distribute the weight as well as on the backs of the arms (read Back of the Arm in Pilates). Also, holding the supported shoulder stand will help you find the lift through the rib cage and out of the hips that you need to make Pilates shoulder stand-based exercises work.