Yeah i used to get that problem. Usually its just a habbit you can't get rid of. Like i get so worked up as to grip the edge of the table during exams... which after 3 hours produces one dead arm... ANYWAY
My advice is GET A GOOD PEN!!
I just bought some Paper Mate, Gel Rollar Ball II retractable ones, and theyre great (go my product whoring)
But seriously having a pen with a nice grip that doesn't drag makes a HUGE difference to the overall well being of your arm.
Just find a pen thats comfortable, with a nice grip, easy to write with, then buy 10 of them and don't look back.
And if all else fails just shake your arm like a mad man being attacked by an invisible pihrana, and ontop of that he has parkinsons. NOTHING beats a good shake.