not sure what its called but I have this little thing that you squeeze to close and the coils make you wokr your wrist muscles. Maybe someone here might no the name of it
There is a many choices for improving wrist strength. Here are a few 1) Rotate your wrist with tennis racket / a big soft drink bottle(I mean BIG, which is 1L and above!!!) 2) Squeezing squash ball or wrist coil. If you can squeeze till the squash ball break, your power is there! 3) Doing double skipping http://www.badmintoncentral.com/forums/showthread.php?t=29922 (1) and (2) can be done when you watch TV. If you are in school, you can do (2) in class.
aside from the rotating the tennis racket you can also get a 12" pipe then put a good size string, rope, or lanyard on it and at the end your desired weight (5 lbs, 10lbs so on)... hold it up and start reeling it up and down... it will work out both your wrists and forearms