Well I have to write 4 pages per essay and I do naturally get the same problem, especially since it is a frequent deed.
Most cramps are brought on by too much pressure being applied to the fingers while gripping the pen, its a common trait that some people find it hard to let go of, but its negative in the sense that if you put too much pressure on your fingers, the pain will work its way through your hand and eventually you'll end up with wrist cramps. The solution to this is to keep you fingers light on the pen and just let your movements glide softly, though I recommend a good pen since most cheap pens almost force you to push the pen down forcefully on the paper.
From personal experience I've noticed that laying your arm flat on the table facing left (if you are right handed) and writing like that is much less painful that doing it with your elbow facing your body or facing diagonal. This is because as your hand moves from left to right, its your arm that is doing the moving. Though I will admit if people are watching, you'll look like a bit of a twat. So if you're okay with that then thats okay. (note that this will eventually tire out your arm, so then you can move back to the elbow facing technique and let your wrist to the work until you've recovered)
Apart from that, all I can say is to pace yourself. The faster you write the more strain you are putting on your wrist, its like when you are sprinting in the park and you are totally knackered after 15 minutes, yet you know you could have gotten to the same point (in more time of course) while walking and you would have more energy. Though this may not be optimal if you are being timed in a test or something. If it leisure writing then it should be okay to put this into effect, take your time.
However in terms of easing pain afterwards, you could always dip your wrist in cold water and allow the low temperature to numb the pain. Though its better to avoid it altogether.