I, being genetically challenged and have to work on strengthening my forearms and wrists to handle the pace and heavy hitters out there.
There are a few things you need to understand prior to working out your forearms.
1. You want to do 10 - 15 reps instead of using to much weight as I've found it easy to injure the forearm and wrists with heavy weights.
2. My forearm muscles tighten up after exercise and it takes 3 days with massage and stretching to loosen the forearm up to a level to play tennis without tightening.
3. Following proper form and squeezing the muscle when contracting will produce faster results then to heavy of weight and jerking any weight.
4. Diet, especially protein, potassium, magnesium, and zinc are all important for muscle development and recovery.
The exercises I do are at bodybuilding.com/exercises and are
Palms-Down Dumbbell Wrist Curl Over A Bench
Palms-Up Dumbbell Wrist Curl Over A Bench
Palms-Vertical Wrist Curl over a Bench, this is a modified exercise I use and works well for top spin and wrist strength for using your wrist to direct the ball.