First, as mentioned above, make a conscience effort to maintain good posture.
Next, stretch the scapula protractors. These include the pec major and minor, serratus anterior, and the anterior deltoids. I would also advice cutting down on the amount of work you put into these muscle groups.
Third, strengthen the scapula retractors. These include the trapezius (upper, middle and lower) and rhomboids by using horizontal pulling movements: face pulls, wide grip overhand rows, seated pulley rows to the chest, inverted rows, and bent over dumbbell raises.
Finally, you should also work to strengthen the lower trapezius fibers in a scapula depression movement, such as scapula dips, as well as the external humeral rotators.
Br