I second raptortrappers method. Just use a weight that makes it difficult and adjust up once it gets easier. You can also do the same thing with a short section of large diameter(3-4") PVC pipe that you grip from the ends with your finger tips and roll up thr weight like you were turning a big door knob. This works the wrist in a angled plane and works the hands and fingers as well.

Both of these will make your forearms feel like they may burst into flames. Don't over do it the first time or you won't be able to lift a cup or work your zipper the next day.

http://www.gripfaq.com/Wrist_Strength_Exercises/

This guy has the wrist roller and some other exercises for the wrist. Start light if you try levering (choke up on the handle or use smaller hammer)or you'll smack yourself with the hammer.

I've not had a wrist injury, but I used to powerlift for years and I did this stuff as auxiliary lifts to improve wrist strength in the bench press and grip strength in deadlift.

Start light and go slow. You want to rebuild not aggravate or reinjure. Good luck!