Bit of mild stretching will help your neck too.
Sit up straight, but not tense, try to keep your shoulders low, and then bend your head slowly as much to the right as you can. Ear towards shoulder. Leave it there for ten seconds, and bring your head in a straight position again. Do the same to the left.
Sit up straight, but not tense, try to keep your shoulders low, and then slowly turn your head, to look as far as possible to your right. Keep that position for ten seconds, and slowly turn your head back. Do the same to the left.
Swinging your arms slowly, when standing up straight can alleviate it too.