Bring one of your elbows across your body, towards the opposite shoulder. Use your other hand to bring your elbow closer to your shoulder. Hold for 10 to 20 seconds, then switch sides.
Alternate method: raise your arm over your head and bend your elbow all the way so your hand is behind your neck. Use your other arm to stabilize your elbow. Hold for 10 to 20 seconds, then switch sides. You should feel either of these stretches in the back of your arm.