Performing a shoulder girdle stretch on a regular basis is a great way to prevent injury and to prepare for physical activity. It's always important to warm-up before doing any activity so you can get more blood flowing to the muscles and to relieve any joint stiffness. Shoulder girdle stretch exercises are particularly important because this joint in particular is prone to having problems if you play a lot of racket sports such as tennis or squash.
One of the best shoulder girdle stretch exercises to perform is simply done standing with your back towards a wall, feet about half a foot away. Next slightly bend the knees and put the arms with the back towards the wall, elbows at the side. Begin pushing the deltoids and arms into the wall while slowly raising the arms above the head as high as possible. Once they have reached above the head, lower them back down to the start and repeat once again. While it may not seem like this movement does all that much, because it a dynamic exercise some muscles will be contracting while others are relaxing. You might not feel it as much as a more passive movement but the results will still be noticeable.
Another easy exercise to do is to lay on the floor with your arms out to the side. Next, take one arm and cross it over the body so that it moves as close to the other arm as possible. While doing so, try as hard as you can to keep both parts of your back flat against the floor. If you begin to see the same side as the arm that's crossed over lift up, you've likely moved the arm too far for your abilities. Take it back slightly until you are sure your full back is on the floor.
So don't neglect these two movements from your routine. They really only take a minute or two to perform but could mean the difference between you continuing to play or becoming sidelined.