No matter how comfortable you might be in your carefully designed and properly adjusted chair, sitting in front of your computer at exactly the right height and position, you should still take short breaks to rest your body and your brain, rather than perform exactly the same activity for hours on end.
A healthy computing routine should include these breaks:
Change your working posture frequently.
Take your hands away from the keyboard, look away from the screen, and breathe deeply four or five times every 10 minutes.
Get out of your chair and stretch your arms and legs three or four times every hour.
Move your chair away from the table; roll your head up, down, left, and right; twist your upper body from the waist; and flex your wrists and fingers every 10 or 15 minutes.
Take a 10- or 15-minute break every couple of hours. Walk away from the computer, go fill your coffee cup (or make a cup of tea), chat with a co-worker, or go look at a bulletin board; anything to provide an excuse to move around and think about something different.
If you job allows it, alternate between computer work and other tasks, in order to use different muscle groups.
If you can, take occasional drinks of water, juice, or other liquids while you work. Not only does this keep your body hydrated and move your hands away from the keyboard, but the need for frequent bathroom breaks gives you an excuse to get up and walk away from the computer.
The need for breaks applies to all kinds of computer use, not just productive work. It's easy to enter a semi-trance state when you're playing a computer game that can keep you going for hours at a time. Because games often require a very limited range of hand movements, they can cause damage even more quickly than most other keyboard-and-mouse activity. Remember to stop and stretch, or take a walk between games; those space aliens will still be there when you get back.