Fix Your Shoulders
Shoulders play a huge role in not only upper body strength, but posture. Shoulders rounded forward will lead to pain as well as affecting your back muscles.
1) Banded Shoulder Stretch: Take a band and anchor it to the top. Grab it with one hand and walk forward to get tension in the band. Turn your hand face up and bring your chest up. Not only will you feel this in your shoulder but your bicep are well.
2) Box Stretch: Drop down on both knees. Place your wrists on the box or elevated surface. Let the box pull your arms overhead. If this is too challenging, reduce the height of the box or the elevated surface. Push your chest to the floor. To make this stretch more difficult, you can completely lie on the floor, having the box pull your arms up.
Both these stretches will help improve your overhead positions!