Don’t focus on just one of your shoulders when you bench press. Odds are, if you’ve been around weights for a while, both of your shoulders are messed up.  Poor posture could also be the culprit which is guilty for making your bench pressing motion come out uneven. Also, if one side of the bar is higher than the other throught your benching motion, you may have a rotator cuff injury on the other side.

A good stretch for the shoulders before working out your upper body can be to place your forearms against a door frame and to lean forwars.  Hold this position for 15 - 20 seconde and then repeat.  You can do this forearm stretch several times each day, not just at the gym.

To strengthen up your rotator cuffs, try lying on your side with a dumbbell in the hand of the arm that’s facing up.  Bend your elbow to around 90 degrees while keeping your elbow against your side.  Rotate your shoulder backwards to make your forearm rise and your knuckles point to the sky.  From here, rotate back down to where your forearm is now parallel to the floor.  Congrats, you just completed one rotator cuff strengthening repetition!  Try to do three sets or 10 - 15 reps for both shoulders.  Rest in between should be 20 seconds up to a minute.

Hopefully, after a few weeks of performing these stretches, you will notice your bench press barbell straighten out when your performing your benching motion.