1. Stay HealthyMaintain a healthy body weight and a good cardiovascular system. An unhealthy body causes stress everywhere. Add that to any environmental stressors and you may have a problem.
2. Stay StrongKeep your wrist, arm, hand and fingers strong. It is harder to overuse something if it is normally worked harder. Strengthen the muscles involved and increase flexibility through stretching.
3. Don't Break the WristLay the outer part of your forearm on a hard surface. Let it rotate inward naturally. Keep your wrist straight. That is the natural wrist position (see image above).
Notice that the palm is at a 30-45 degree angle and that the fingers are curled. Keep that position whenever possible. Flexing and twisting of the wrist causes all the tendons and nerves to rub over leverage points at the joints which can cause a lot of problems.
4. Use Your MusclesControl the movement of your hand and fingers through muscle use not tendon/ligament use.
One big problem with typing on modern keyboards is the lack of strength needed to press a key. This causes you to simply start a motion of the finger and let momentum carry it through. This can cause minor hyper extensions and wear and tear on the tendons and nerves.
Musicians are prone to this as well, due to the speeds they need to achieve. Developing strong fast twitch muscles is a better alternative.
5. Take BreaksTake regular breaks to relieve stress. Take this opportunity to stretch and increase blood flow. You should break for at least 10 minutes for every hour of continuous work with 30 second micro breaks every 10 minutes. Performing a warm up and cool down stretch will help as well.
6. Change PositionsChange your position and posture regularly. Change of position will call in different muscles, kind of like a relief pitcher, letting the first group rest.
7. Get a Good GripUse a proper sized grip for your hand.
Look at your natural wrist position again. Now bring your thumb and fingers together until they are separated by the width of two quarters. That is your grip size for holding things. That is your ideal grip for things like handrails or screw guns.
Now continue to close your hand until the thumb overlays the first joint of your index finger. That is your grip size for manipulating things with your wrists, things like hammers, shovels or golf clubs.
8. Maintain Your DistanceWhen working with your hands keep them in the middle ground, not too far, but not too close to your body. This allows muscles in your arms, shoulders and trunk to help share the load.
It also keeps your joints in the middle of their range of motion, which increases blood flow and reduces the flex of tendons/ligaments/nerves over those leverage points at the joints.
9. Don't Go to ExtremesJust because your joints can pivot that far does not mean yours should. Do not flex your joints to the edges of your range of motion while working.
Most muscles can not maintain control of your body at these extremes which can lead to hyper extension and muscle pulls. It also flexes the tendons and nerves over those leverage points of the joints. For more on range of motion see this discussion on body links.
10. The Low DownDo not flex upward. The hand is designed to grip, so most muscle control and joint range is aimed at a downward flex. There's less leverage on an upward flex so the body has to work harder to move that way. The tendons and nerves also have harder leverage points to stretch over.
Keep palms and fingers somewhere between flat and the grip position.
Keep your typing and mouse click upstrokes as short as possible. Do not use the scroll wheel as that motion is almost entirely upward flexing.