you can do push ups on the back of your hands. instead of on your palms, turn your hands over and so that the fingers face each other.
you can also imagine like you have a wet blanket, towel, etc. you imagine wringing it out. go back and forth, turning your wrists away from you, then towards you. i don't think i've done a good job of explaining that one so if your confused, scratch it.
heavy staff work (no, not that staff) will work wonders for total, all around strengthening of the wrists. even if the staff isn't that heavy, your wrists will get a good work out if you practice enough.
i'm nothing to brag about, but i've got pretty strong wrists and a pretty strong grip and have been told this by cats who out weigh me by 50 pounds.
here are 2 exercises i practice often and enjoy cuz they can be done almost anywhere....if you don't mind people looking at you like you're some sort of maniac; i practice an exercise where i start with my hands open, fingers spread apart as far as possible. then, i slowly make a fist, imagining squeezing a ball as hard and as slow as i can , until they are clenched. then i relax, let them open and then spread the fingers as far apart as possible again. the slower this is done, the better. this will improve your grip, forearms and naturally, your wrists. do as many as possible. after 30, my arms from my elbow down are usually on fire. my goal is 100 in one sitting. you don't want to keep a clenched fist for more than an instant....in other words, your hands should never 'hold' a position.
also, to improve how fast you can make a fist (which also aids in grabbing something quickly with power), make a fist and open your hand as many times as possible in 30 seconds. remember not to cheat....make a complete fist (strong) and open it completely. these few exercises have worked amazingly for me.
lastly, stretch your wrists before and after exercising them, but i think as a martial artist, you know the importance of stretching a muscle before using it.
good luck.
p.s.- i hate people who give 'dead fish' handshakes.