Get a "thick" piece of pvc pipe ( 20 inches long ) and then drill a hole right through it (in the center). Tie a piece of nylon rope through it ( one that will obviously fit through the hole ) and then tie the other end of it to a weight (I used a 7 1/2 pounder)... Get into a sitting position and roll the weight up to you ( as shown in the photo below ). As you do this, place your forearms on your legs ( as you are in a sitting position ).

You simply rest your forearms on your knee's and roll up the weight. You can either drop it back down to the floor, or slowly release it back to the floor, what ever suits your fancy. I do this about 3 times a week and only do about 10 repititions. It has helped me develop much stronger wrists. I used to sprain them all the time in Krav Maga, that no longer happens. ....Anyhoo, it works for me.