This article is from the Ergonomics and Typing Injury FAQ, by Dan Wallach firstname.lastname@example.org with numerous contributions by others.
[I so liked the way this was written in the New Zealand book that I'm lifting
it almost verbatim from Appendix 10.]
* Let your shoulders relax.
* Let your elbows swing free.
* Keep your wrists straight.
* Pull your chin in to look down - don't flop your head forward.
* Keep the hollow in the base of your spine.
* Try leaning back in the chair.
* Don't slouch or slump forward.
* Alter your posture from time to time.
* Every 20 minutes, get up and bend your spine backward.
Set the seat height, first. Your feet should be flat on the floor. There should
be no undue pressure on the underside of your thighs near the knees, and your
thighs should not slope too much.
Now, draw yourself up to your desk and see that its height is comfortable to
work at. If you are short, this may be impossible. The beest remedy is to raise
the seat height and prevent your legs from dangling by using a footrest.
Now, adjust the backrest height so that your buttocks fit into the space
between the backrest and the seat pan. The backrest should support you in the
hollow of your back, so adjust its tilt to give firm support in this area.
If you operate a keyboard, you will be able to spend more time leaning back, so
experiment with a chair with a taller backrest, if available.
[Now, I diverge a little from the text]
A good chair makes a big difference. If you don't like your chair, go find a
better one. You really want adjustments for height, back angle, back height,
and maybe even seat tilt. Most arm rests seem to get in the way, although some
more expensive chairs have height adjustable arm rests which you can also
rotate out of the way. You should find a good store and play with all these
chairs - pick one that's right for you. In the San Francisco Bay Area, I highly
recommend "Just Chairs." The name says it all.