These exercises that charlierfederer posted a few times include some wrist work..
http://www.asmi.org/SportsMed/media/thrower10.swf
Other than that, try some regular static stretches (warm up your arm first) and focus on your technique when playing, try and keep your wrists as loose/relaxed as possible, they should not be tense at all, so loose that if you relaxed any more the racquet would fly out of your hand.
Might be worth doing some drills against a wall or with a partner to get that set so you don't tense up during play (AFAIK the wrists are not be used as a form of muscle power during play, just a relaxed part of the 'kinetic chain')
I had a bad wrist when I got back into playing a few months ago (after years of not playing at all), so much so it would hurt when I hit a volley and especially on a mis-hit (made me want to drop the racquet it hurt so much).
I worked on relaxing my wrists and I wore a wrist brace for a while and it went away (after about a week or so)
Most of what I do in terms of stretching comes from stuff I learned for music, but from seeing a physio and reading books and stretching the same stuff applies so I still do the same stretches.
Try this youtube vid by a sax player who suffered from tendonitis, the stretches he shows you how to do should help you avoid wrist and tennis elbow issues (there are some more in his videos too.) These are 'static' stretches, try and warm up your arm a bit before doing them (jog on the spot, go for a walk or get a resistance band and try doing some light work with that first.)
http://www.youtube.com/watch?v=MXwfabQdcck
Good luck.