Here is a great video, looking into how to reset bad shoulder position. Literally, EVERYONE reading this should take the 5 minutes to watch this video and think a little bit more about how to fix their shoulders. Be ready to see some of this coming to a warmup near you soon!
Roll: Lay on your stomach, put your arms above your head. Lift your right arm to the side and then up, using the weight to help you roll over one or two times. Then use your left arm to roll back the other direction. Focus on pulling yourself over with the weight of your arm rather than pushing off the ground.
Rock: Get on your hands and knees, and gently rock back and forth pushing your hips down towards your feet and then rocking slowly forward. You can also do head nods in this position by bringing your chin to your chest and back up. This loosens up the muscles in your neck and upper back.
Crawl: On all fours, crawl forward a few steps and then back. You can exaggerate the hip movement towards the ground as much as feels comfortable to stretch your hips. You can look side to side as you crawl backwards to stretch your neck.