Also:
- The top of the monitor should be level with your eyes.
- The monitor should be right in front of you, at three feet distance (1 meter, if you prefer).
- Turn off the monitor, and look at it. Can you see any bright reflection, like windows, lamps, or *shudder* the sun ? Do something to eliminate those reflections. They are a big cause of eyestrain.
- What do you have behind the monitor ? There should be something of comparable brightness, like a uniformly lit wall. Neither a pit of darkness nor a very bright scene (like the outside world in daylight) will be good for your eyes.
One very good position for the monitor (that fixes 3 and 4) is with the
screen surface perpendicular to your window panes (if you have a window), or
parallel to the light fixtures (if you have neon tubes).
Unique situations will require specific solutions: for example, in a room lit by powerful yet indirect ligthing (like a
halogen bounced off the ceiling) you may be better off placing something dark
behind you, in order not to have a white wall reflecting in your
screen.
Monitor placement is important, but it is just a part of workstation ergonomics. Be careful how you sit: your back will last longer.
Oh, and while you are at it, throw away those cheapass mosquito netting screen overlays: they only reduce contrast. The only overlays worth having, and only in special conditions, are the heavy dark crystal ones.