I use dumbells for my whole arm, but that's cuz I also play tennis but my suggestion to you is to also try those squeeze things (totally forgot the name of it) it's like two bar that you squeeze together works wonders on your grip and thus your forearm.
called handgrips. Good for finger, hand and forearm strengthening. Dumbells are the most effective things for wrists and forearm strengthening.
How heavy should the dumbells be?
Which excercises should I do with the dumbell's to gain the best outcomes?
Construct your own equipment!
Get these things: a soft-drink bottle (the larger the capacity the better, min 1.5 litres), a wooden/plastic pole about 30cm long, nylon string about 1.2m long.
Fill the soft-drink bottle up fully with water. Drill a hole (large enough for the string to thread through) in the centre of the pole. Thread the string through and tie it up firmly. Tie the other end of the string to the neck of the bottle.
The two ends of this pole will form the handles of this equipment. With both full arms' stretch in front of you, hold the handles to move the bottle off the ground. Now, lift the bottle up by twisting the pole using your hands in any clockwise/counter-clockwise direction you prefer, always keeping the handles at level full arm's stretch in front of you.
Keep repeating until your arms get tired. Sooner or later, you'll get your wrists toned up for more wrist power. Increase the load or change the grip surfaces on the handles if necessary.
what weight should you use...hmmmm...depends on your strength lol, what i think is the best is to hold it and curl it like you're starting a motorcycle and do that like 15 times slowly, then do the reverse motion, (often i like to do both hands so you don't have an over developed dominate arm)
but as for mass, i'm like 180 and i normally don't like to go over 40 lbs
the best, easiest and most comfortable way for your racketgripping is probably this:
http://www.youtube.com/watch?v=YLlYpja1nRo
I have done it a few weeks, and not even every day. And I felt a difference already.
i have a fren who is ex-national backup player...
he has very strong wrist because he use a handfan to grill Satay for his father stall
everyday 4-5hours...
Yep this , and the figure of 8's will make a vast significance to your wrist strength.
Do this regurlarly , and you will see your veins pop up LOL!