Most people assume that a desk is a desk is a desk. However, nothing could be farther from the truth (unless your desk was a duck). When you're sat at your perfectly supporting chair, feeling like you have the posture of a teenaged supermodel, the whole effect will rapidly unwind if your desk is too high or too low. At a bare minimum, your desk should be height adjustable so that you can match it to your chair. Although you might think that merely adjusting your chair to your desk height would be enough, having both adjustable allows you to find exactly the right position in which to spend your eight hour day.
Once you've got your body at the right height, consider your legs. Your feet should be planted firmly on the floor, or alternatively on a foot rest attached to your desk for maximum stability. Having your legs swinging will put too much strain on your lower back and could lead to sciatica.
Your desk may be covered with paperwork and photos of your dog, but the desk surface itself can cause problems. Positioning a light directly above it can cause glare if the surface is glossy, and to avoid problems a desk surface should always be matte. Also think about the other accessories on your desk - do you have a document holder? If you regularly flick between documents and your computer screen, put a document holder next to the screen so that your eyes aren't constantly having to refocus.
Probably the most used equipment on your desk is your computer, so positioning this properly is key. Although it's tempting to simply plonk your keyboard directly in front of you, consider exactly how much of the keyboard you are using. If you never use the number pad, move the keyboard to the right so that the main letters are directly in front of you. Even better, get a keyboard without a number pad so that you can place the mouse where it would otherwise be. Placing your mouse too far back on a desk is a common problem, and one that can lead to back and shoulder problems as you hold your arm unsupported while using the mouse. Ideally use a cordless mouse to avoid cable tangles and any issues positioning the little rodent exactly where you want it.
It almost goes without saying that you should be sitting straight-on to your desk, but for those who regularly like to dawdle or doodle at an angle, your back will thank me for reminding you.