Wrist curls with a barbell will do the job. Try a set with your palm facing down and facing up.
Also, rope lift with a weight hanging at the end of the rope will give your wrists a really good racquet.
Squeezing a tennis ball is good too.
On volleys, I found the best tip was to move your whole upper body as one unit. For the best volley, your arms really shouldn't be moving. When you turn your shoulders, your arms will automatically turn towards the ball. And make sure that your non-racquet hand is moving along with your racquet.
And if you have time, watch the ball hit your strings. It's good practice.
There's also a chance that your racquet might be a bit too heavy for you. Don't let the grams on a racquet fool you, I tried out two racquets-one 324g and other 354g, although the extra 30g's might not sound a lot I ended up having a much worst game especially after lugging that thing all over the court for a few hours making the racquet unplayable for me.
As for the volleying, you can always position your hand a little higher towards the throat of the racquet, you'll lose a bit of reach but if you're footwork/speed is good, you'll gain more stability and impact of the ball will not affect you as much.
Two other things you might consider is that
1) Your grip size might be wrong for you. If it's too small, your racket could slip. If it's too big, you'll have trouble holding onto it. A decent pro at an indoor tennis facility or a tennis specialty shop should be able to help with this.
2) Check to see if you have a head heavy racket. That can mean all the difference. Even a light racket can kill you if it's made for baseline bombing.
You might want to try the racket finder at Tennis.com - Gear. Look up the stats on your stick, and see how it compares.
Focus on swingweight and balance as well as overall weight. You probably need a racket that has most of it's weight in the grip end. (low/light swingweight, low/light balance).
Of course, what I'm talking about is buying new gear. If you have the kind of job in which carpel tunnel syndrome and bad posture are the most significant injury concerns, you might simply have weak wrists from all the typing, mousing, ten-key, etc. What these other guys mentioned can help a ton as well, and cost you very little.
I develop training aids for tennis. I have a device that is designed to strengthen the wrist and arm muscles while you warm up to develop muscle memory,endurance and racquet speed. All to make you a better tennis player. It is called ProTrainer WinSock and you can see it at www.ptwinsock.com. Please share my website with others.