Instructions
Get in shape. The stronger and more flexible you are, the less likely you are to injure yourself. Low-weight strength training will make you stronger without being too hard on your knees. Focus particularly on ankle and calf exercises.
Practice and become familiar with the most common snowboarding moves before you go out on the slopes. Get comfortable with the correct stance -- fluid center, straight back and knees slightly bent -- while alternating putting your weight to both the front and back of the board. The more muscle memory you have, the more likely you are to make the right move at the right time.
Learn to fall uphill. Falling is inevitable, and if you can fall uphill, you will lessen the impact. If you can't help but fall downhill, try to relax and roll loosely rather than use your arms and legs for leverage.
Keep your hands clenched into fists while you are riding. If you fall, you will punch the ground rather than take all the weight on your wrist and risk a break.
Get official instruction. Doing the stances and moves correctly can reduce your risk of injury. Learning -- or improving -- under the eye of an expert will streamline your training and ensure that you are practicing and executing moves properly and safely.
Stick to your skill level. Make sure you are on hills and areas that are appropriate for your skill level. Although pushing yourself to improve is important, you are much more likely to get injured if you find yourself in an area that you cannot navigate safely.