"More people are showing up at eye appointments complaining of headaches, fatigue, blurred vision and neck pain—all symptoms of computer-vision syndrome (CVS), which affects some 90% of the people who spent three hours or more at day at a computer, according to the National Institute of Occupational Safety and Healthy."
The above quote comes from a Wall Street Journal article published yesterday. If this is true, the only reason the percentage is so high is because people don't know the simple steps to avoid these symptoms. Here are a few things I do to make sure I never have to leave my computer...unless I really want to.
Adjust Monitor Brightness
I think the single largest reason why people have trouble sitting in front of a computer screen for an extended period of time is because their screen is too bright. Most monitors are set to be way too bright by default. Try turning the brightness all the way down. It will make your eyes think you're looking at a piece of paper rather than staring into a flashlight. (Make sure you adjust your brightness and not your contrast.)
Ambient Light
I like to keep my office pretty dark, but it's important the ambient light in the room doesn't contrast too greatly with the physical world. For example, if my monitor is bright and my room is dark, it's going to hurt my eyes because of the stark contrast. This is why your parents always used to tell you to watch TV with a light on (at least my parents did). But an environment that's too bright isn't great either. When I used to sit in a cube all day, I had the florescents above me ripped out. Distancing yourself from ambient light in a close proximity is a good idea.
Monitor Height
A lot of times, neck pain is caused from leaning down to look at a computer screen (this happens a lot when working on a laptop). Whenever possible, make sure your monitor is at your eye level. I would suggest that if you're looking straight out from your chair, at least 1/3 of your monitor should be above your eye level. I used to set my monitors on books. Now I use actual monitor stands that get my monitors off my desk by about 4 inches. Avoiding looking down will help reduce neck strain.
Back Support & Sitting Position
I never work in front of a computer without a pillow behind my lower back. Even actual back supports from office supply stores don't work well. I recommend an actual pillow. It helps keep the upper body back and upright. It also helps keep my head back to where I sometimes rest it on the back of my chair. This reinforces the last idea to keep your head looking straight on rather than bending forward and looking down.
Even Multiple Monitors Help!
This isn't for everybody, but...I work in front of an array of 3 or 4 very large monitors. I think doing so actually helps keep my eyes fresh and my neck from hurting, because I have to physically turn my head to the left and right to see all my screens. This keeps me from focusing in a single, small area for too long at any given time, and it also reminds me to blink when I turn my head.
These suggestions definitely aren't scientific - they're just a few bits of common sense that I've realized over my time using computers. Hope this helps keep you pain-free!