Turn your head all the way to the left and hold. You can add a little more stretch by pushing your chin with your fingertips. Repeat for the right side.
Pretend you are looking at a spot of mustard on your shirt. Flex your neck to bring your chin as close to your chest as you can go and hold.
Lateral Neck Stretch
Looking straight ahead, cock your head to the left as if you were trying to bring your left ear to your shoulder (don't cheat by shrugging your shoulder up to meet your ear). Repeat for the right side.