Two suggestions: get a differently shaped or weighted pen. If I use a pen with too narrow a barrel, I get a cramp. Or if it's too short.

And the other suggestion which has done the most to alleviate cramping and carpal tunnel inflammation: switch your mouse to your non-dominant hand.

The dr. who was treating me for a trauma sprain in my right hand suggested that. It splits the work between your two hands -- one for writing, one for mousing -- and lessens the strain on your dominant hand.