You want to see instant magic ?
It's as much a posture issue as well , get your ass onto a threadmill and start walking backwards holding onto the side rails till you get your balance and rythum ... ( this is really cool )
Then drop your hands down and you'll naturally pull back your shoulders without even knowing it inorder to maintain balance ... i've done this with 100 clients in the past and said nothing . When you get off the threadmill you'll notice your now holding your shoulders back untill you slip back into your bad posture position .
Posterior deltoid work will also help as muscles are tight when worked so they will help pull back the shoulder ... any type of high row to the shoulders will work .
Reverse your body position on a pec deck and fly backwards
High cable row with elbow high and level with the shoulders
Same thing can be accomplished with bent over dumbell rows if you row high to the shoulders
What your after is to tighten up the back of the shoulders in an attempt to drawn the scapula back and keep it there in the relaxed position