Firstly, choose material wisely. Use a comfortable chair, and set the height so that your elbows and knees form right angles. And choose a style with arm rests.
Ideally, your computer screen should be at about fifty centimetres from your face. When sitting, the top of the screen should be in a straight line from your gaze, slightly tilted backwards.
The keyboard should be a natural extension of your fingers. The desk should be deep enough so that you can rest your forearms. If this isn't so, you should opt for a wrist rest in front of the keyboard to avoid carpian canal syndrome. The mouse should be as close to the keyboard as possible.
Lighting is also important. In general, prefer natural light. As soon as light fades, switch on a desk lamp and place it by the screen to avoid glare. Lastly, and this is the ultimate luxury, when you feel tense, indulge in fifteen minutes of total relaxation with a sit-down massage given by a specialised company.