Quick and dirty post because some folks are in need of this info fast; i'll tidy it up later.
Firstly, watch this video and follow the instructions:
http://www.youtube.com/watch?feature=player_embedded&v=hUyMNyrOHJQ(feel free to check out related videos and do what those say too, but i only just do the exercises from this one.)
Okay now for instructions from one of my hobbies of research, acupressure massage. aside from chi points that you may or may not bother to believe in, you're going to be massaging tendons and muscles and it's going to hurt. no pain, no recovery. I personally learned the following steps from an acupressure massage therapist who goes to my grandparents' home every week to work on my grandfather, who's currently in the middle of recovering from a stroke he had last year.
1 - lay your hand/wrist on the desk in front of you, palm up, relaxed.
2 - you see the crook of your elbow? that solid line of wrinkle from bending yuor elbow? find the center of it, then a point above it (towards your shoulder) about 1-1.5 inch away.
3 - from there, find 2 points on either side of that (left/right), bout 1 inch away (maybe a little more, if chances are you're a dude and/or your arm's bigger than my flimsy twigs).
4 - dig your thumb (or knuckle, those work well too) into those areas, massaging them out. one, then the other. however long you want, better to inflict as much pain on yourself as you can tolerate. but don't drill a hole through your arm please.
5 - do the same for below the crook of your elbow (towards your wrist). the point on the same side as your pinky finger may be a little harder to find - you might have to press deeper into your arm or move the point around a bit, but you know you've got it once you're feeling pain.
6 - and this one you MUST do last - go back to the center point between the two you massaged for step 5. if you did things right it may be a bit swollen now. massage that point.
7 - you can also lift your arm off the flat surface now, and massage the tendon in your wrist below your pinky finger. for me personally that's a strained spot because of how i rest my hand on the desk when typing or using my mouse.
now you can lay your arm on your lap or whatever, and massage the top of your forearm close to the elbow. work your way towards the wrist i guess.
finishing up, grasp your drawing wrist firmly with your other hand, and rotate your drawing hand a few times to sort of realign your wrist bones. i also like to give my knuckles a squeeze to try and get rid of some tension that's built up around them.
I don't draw as much as you guys, but i grip my pencil really hard and i
have had wrist pains, so doing these exercises has really helped to keep it to a minimum.