best off not doing weights to work on this. weight lifters will activate the big muscles to pull the weight. you want to neurologically activate the smaller muscles that are getting neglected w/less stimulation than they need to maintain a balanced posture. flexion dominates our lifestyles and we need more extension.
try this exercise ... stand against a wall, back flat against wall, arms at sides, palms up/supinated. keep back flat against wall while raising arms to overhead, while keeping elbows against wall. kinda like a jumping jack (upper body motion only), or snow angel action. do it a couple times week for three sets of 10 each time.