Lot's of possible causes / influences for this, as already highlighted in previous posts in this thread. I think vice-like grip is usually the main culprit, exacerbated by boom size/grip, conditions, wetsuit tightness etc.
Arm conditioning is important, but none of us can be bothered with that, and once the condition starts it's difficult to get rid of while you're windsurfing regularly.
I got one of these a few weeks ago - 4Arm Strong device. Yes it looks like a torture device, and it's rather spendy (although much cheaper than a new boom), but it really seems to work for me. Ordered mine online from a shop in UK. You don't wear it while you're windsurfing btw, it's for focused stretching of the forearm muscles.
No, I'm not affiliated with the company