Even if you are a careful rider, you can only be as safe as your safety equipment. When you skateboard, you should always wear closed-toed skateboard shoes to protect your feet. Wear a protective helmet and knee and elbow pads to reduce your chances of being seriously injured. You may want to consider getting other safety gear such as padded shorts and wrist guards.
Be sure that your board is suited for the type of riding you are doing. Slalom, freestyle and speed boards are designed differently, and each type should only be used for its intended purpose. You should also make sure your board is rated for your weight. Finally, always check out your board before you ride. Look for cracks in the deck or the wheels and sharp or jagged edges. Also make sure that the deck has good traction on it for your feet. If anything is wrong with your board, don't ride.
Watching the pros ride, its easy to be temped to try tricks that are over your head, but remember that those pros had to ride for years before they could do those stunts. Don't do tricks too difficult for you. Start slowly and build to more complicated tricks. When you are pushing the limits of your abilities, be sure that you're wearing protective gear. You will fall at some point, so you should know how to fall right. If you feel yourself losing balance, crouch low on your skateboard. Try to absorb the impact of the fall on a soft part of your body, such as your shoulder. Don't try to stop yourself when you fall. Instead, roll with the fall to avoid injury.
Be careful around cars. Street riding can be fatal for skateboarders. The U.S. Consumer Product Safety Commission recommends that skateboarders never ride in the street. If you choose to skate in the street, avoid areas with high traffic and wear reflective clothing. If you are doing tricks, do them in a skate park if one is available. But if you are in a skate park, don't assume the terrain is safe. Always check the area you are riding in for potholes, loose equipment and other hazards.