Hey man, I wouldn't worry too much about your wrists feeling stiff or sore. If your wrists do start to get sore always make sure that you stop playing because you can damage your wrists quite badly.

Before I play I always warm up my wrists. Here is a video on some hand stretches, I couldn't find the one I was looking for but this is still quite good.

http://www.youtube.com/watch?v=8I63ld7wZCs

Warm up for 5 to 10 minutes by doing the stretches shown in the video and and move your fingers around as if you were playing the guitar or piano without actually using the instrument.

Hope this was helpful