Hi Alice it is always a good idea to try to warm up gently before putting your full weight on your wrists and it also important to build up strength in the muscles surrounding the wrists and you can do this by doing easy crow pose. I have attached a video below for crow pose. Also try to spread your fingers and grip the floor when you are in a pose as this will protect your wrists. Hope you find these tips helpful. I have included a couple of wrist stretch videos that you could do before you do your yoga. Enjoy your practice!
You can also view Kevin Fong's video on wrist and elbow pain