Here's my favorite wrist prehab/rehab exercise:
Grab a 10lb sledgehammer by the end. Take a pretty narrow stance, then with a straight arm, simply swing your arm back in forth to make arcs that are about 120 degrees. Be sure you don't clock yourself in the ankle with the hammer. Once you can do that without fear of smacking yourself, you can let the hammer swing through to vertical in the front of the movement (bending your arm then makes it easier), then balance it for a few seconds with your grip and wrist movement before letting it drop into the next arc. Work for 25-50 reps per hand, for one or two sets.
Call me crazy, but it's good. It's a good combination of mobility work, high reps for bloodflow, smooth flowing movement (not at all like smacking something with a sledge), wrist traction (especially as the weight swings through the bottom), and fine motor coordination (balancing at the top). The only downside is the whole risky business of swinging a hammer around. Honestly though, if it lets a 200lb+ guy do handstands without hurting his wrists, it has to have some merit.
I'm a fan of the forearm roller too, but the hammer is better for (my) wrist injuries.
And yes, I'm actually holding that handstand. Get on my level.