1. The top of the monitor should be level with your eyes.
  2. The monitor should be right in front of you, at three feet distance (1 meter, if you prefer).
  3. 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.
  4. 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.