There are some basic guidlines that you can use in order to limit or eliminate the stresses that come with computer use.  This guide is BASIC but easy to use and can help sort out your ergonomic issues.

1)  First of all, your seating posture has to be addressed:

Good posture is essential to prevent injuries.  Starting from the bottom, your knees should be bent 90 degrees with your feet flat on the floor   and your thighs resting comfortably on the seat.  Your hips should be open slightly up to 115 degrees allowing you to recline slightly into your backrest.  Your upper arms should hang comfortably below your shoulders and your ears should be aligned vertically with your shoulders.  Your elbows should be angled between 90 and 110 degrees and your wrists should be flat level with your keyboard and mouse.

2) Proper mouse placement will prevent excessive stress on your wrist, elbow, shoulder, neck and back:

The Ergo Nav was specifically designed to help reduce the stress associated with mouse use!

3)  Proper monitor position helps prevent slouching and eye strain:

When gazing straight ahead your eyes should focus on the top 1/3 of your monitor.

4)  Lighting is important.  Lighting should not produce glares or make you squint.

5)  Watch your reach zones!  Items that you use on a regular basis should be within easy reach and not require any stretching or over-reaching.