clipped from skateboard.about.com

The biggest thing is, when you fall, try to NOT use your hands to catch yourself. This might be kind of hard to learn, but if you lose your board and you are going to smash into the ground, you should try and let your shoulder and body take it, rolling with the blow as much as you can. Catching yourself with your hand is a great way to break a wrist, and while wearing wrist guards can protect you from this, it's dangerous to get used to using your hands, because at some point you will skate without the wrist guards ...