Attach a piece of rubber tubing to a door handle and grasp the other end with the working hand. (If you're more comfortable wrapping it around your hand a few times, that's fine.) Your hand should form a fist, knuckles facing up. If the tubing is attached to a door know on the left side of your body, use your right hand to do the exercise. When the tubing is attached to a door knob on the right side of your body, train your left wrist.

Now flex the wrist, working against the resistance of the rubber tubing (flexing in this case means moving the knuckles in the direction of the elbow). Repeat for 10 repetitions. Before changing hands, change the position of your body so that you're now facing in the opposite direction. (The tubing is now on the same side of the body as the working hand). Now extend the wrist, working against the resistance of the rubber tubing (extending in this case means moving the knuckles away from the elbow in a "bent wrist" position), the exact opposite of the "flexing" you did before. Repeat for 10 repetitions and then change hands.

Alternately, you can use hand weights to accomplish similar strengthening. Take a pair of light dumbbells, one in each hand, get in a comfortable seated position and rest one arm on each leg with the wrist and dumbbell hanging just off the knee. Start with the palms facing down. Now "curl" the wrist, flexing so that the knuckles move up, in the direction of your face. Repeat for 10 reps.