What health and ergonomic factors should an all-day computer user take into account?

Much of this may not be possible because we don't have total control over our workspaces.

• sit up straight
• get a good chair
• get a good desk and keep it clean
• lay your keyboard and mouse out where you can get and use them without reaching or stretching
• put it all in a quiet location
• make sure there are no harsh, loud lights or visual distractions
• ensure the computer is quiet or silent
• get a display that doesn't flicker
• set the display size to something you can read without straining
• make sure the display is at a level/orientation that you can look at without straining
• get a keyboard and mouse you are comfortable with -- I'm not a fan of "ergonomic" keyboards, but if you like them and can use them, great. The bottom line is: If it hurts, STOP USING IT.
• same for wrist-rests and the like -- if you like them and they work for you, get one that you can use. I find my wrists are less likely to hurt if I am forced to keep them off the available rests.

If you get into a situation that hurts, tell your boss. In many juristictions you can make it a workman's-compensation or job health-and-safety issue if it is a real problem.

Sit up straight!!

If you don't have an adjustable and comfortable chair, buy one yourself if your company refuses to provide one for you, though I would probably leave a company like that.

Pick a comfortable keyboard with a handrest.

If you have a second screen, don't keep it on the same side all the time, or put it above your normal screen or you will eventually develop neckpains due to always looking to the side in the same direction.

Try to do some hand exercises like these if you get pains in your wrist.

If you have a raisable desk, stand at your desk instead of sitting a day a week or so.

Get a soft movable wrist rest for your mouse, movable so that you can place it under your elbow if you start feeling pain there.

Keep your monitor at the right height so you're sitting somewhat straight when you're in your chair.

Also, if you have glasses, there are certain coatings for the glass (like anti-reflective) that makes working with a computer all day easier.

Make good use of your company massage therapist and ergonomist if they have any.