Have Two Screens
The only advice I feel qualified to give is this one. Having two screens is immensely better.
Matter of preference. I prefer a single very large, high res screen (30", 2560x1600). I tried using several screens, but my neck started hurting from looking left and right. Also, vertical space is important - a screen with 1920 pixels doesn't use up my eyes' vertical angle (where I can look comfortably without moving my head).
Continuation of: Spend Money on Ergonomics, by Kevin
Three years have elapsed since Kevin wisely told us to spend money on treating our bodies well. It may be time to check for new gadgets, to verify what has worked and what has not etc...
If you have purchased an item for this purpose, or intend to buy one and don't know which, tell here, ask here.
Nick Bostrom uses a mouse that looks like a plane controller joystick.
I've seen keyboards that bend sideways, that are concave, that are convex, and that look like a sphere.
At FHI, dozens of books are used so that computer screens stay at eye level or above.
But I am no expert and I have not looked myself, nor would know how to. So please share in the comments the best knowledge about:
Keyboards
Mice
Chairs
Balls to sit on
Pillows
Beds/Matresses etc..
Screens - Size, position, brightness etc...
Other household office items - Stairs, Handles, Shower etc...