Chest out, spine erect, abdomen in and your shoulders will automatically move back into their proper position. One of the first exercises I would give my clients back in the day was merely to walk erect and sit erect all day, every day. Believe it or not it made a HYUGE difference.
To learn proper posture one must be *re-acquainted* with their TVA muscle. Does anyone here know how to properly do that? One can't expect a client to engage a muscle that they can not feel.