Increase Lower Arm Strength for Control

To increase your lower arm strength, you don't need to buy a fancy weight machine.

Just take a wooden dowel (like a foot-long piece of a broom stick) and tie a heavy string to the center of it. Then, tie a weight to the other end of the string. Hold the dowel in both hands and gradually raise the weight up by rolling the string around the dowel, gripping and twisting your hands alternately.
Here's how the exercise looks.

Do this a few times and you'll feel the burn.
Do this regularly, and you'll feel the increased control you now have over your racquet and your shots!