strenghten your rear delts and mid traps, this well help pull your shoulders back, without seeing you I would be willing to bet this is the problem, also make sure and stretch your chest out at least 3 times a week for a while, this made a huge difference with me. If you need examples of exercises here you go:

http://www.abcbodybuilding.com/exerc...dsupported.htm

http://www.abcbodybuilding.com/exerc...rdeltraise.htm

http://www.abcbodybuilding.com/excer...20laterals.htm

I can not find any for the mid traps but if you stand facing away from the lat pulldown machine and grip the bar with an overhand grip, easier if you have someone pull it down to mid back level for you, then press down directly to the floor and concentrate on your mid trap which runs below your upper trap. You will only want to use 45 pounds or so in the begining as the exercise is ackward for a while.