From LifeHacker: Loosen Up Your Writing Grip to Banish Pain.
If your hand gets cramped and sore when you write long hand try to give up your death grip on the pen. Don’t carve your words onto the page. If you work on not pressing down so hard or squeezing the pen like a hungry python, your hand will not tense up and you will be able to cure your writer’s cramp. I’ve found it does work, though it isn’t easy to get used to writing long hand again. I do like taking paper and pen out to the local coffee shop and just letting ideas come to me when I’m not looking for them.