Creating awareness of chest and shoulder placement in preparation for shoulderstand:
Sitting at the edge of your chair, reach around and grab the sides of the chair back. You might find your shoulders shrugging up to your ears. Take the shoulders down away from the ears, lengthen your neck. Take your shoulderblades down your back (depression), and the lower tips moving in (imagine them moving to your front ribs, with out poking the lower front ribs out.) See if you can move the hands up the chair back, without losing the shoulders moving down. Drop your chin towards your chest - or rather, raise your breastbone towards the chin. You should be getting a feeling of the chest opening, and the outer shoulders rolling open.
Once the body has become aware of the position of chest and shoulders in the upright position, try to recreate the feeling in bridge, moving to recreating it when in shoulderstand.