stretch your pecs and all the muscles around the shoulder girdle. stretch your TRAPS AND LATS as well, most people suffer from tightness here, especially if they have lifted in powerlifting type movements. you probably need to strengthen your upper back and rotator cuff as well. this can be done with all the usual external rotation exercises, and sotts presses, bent over rows, scapular pushups, and chinups. make sure your shoulderblades are tucked together at all times whilst doing these exercises.