First and foremost if you are working in an office or study is to buy yourself a well designed ergonomic chair which should be placed close to your work surface. This site has a number to choose from http://www.back2.co.uk/Ergonomic-Chairs/Office-Chairs-c10/?gclid=CO2opPGwz6oCFUVO4QodnRHr0Q
With a lot of us using laptops nowadays it does mean that we move around a little more but it is still essential to have your laptops at the correct height as well as your computer.
Your keyboard should permit your arms to slope downwards slightly, with your hands, wrists, and fore arms straight, in-line and roughly parallel to the floor.
Your screen should be 14-25 from your eyes, depending on your vision.
The centre of your screen should be level with your chin, or about ten degrees below your eye level.
Let your feet sit flat on the floor and not tucked under your chair. Keep your shoulders and back relaxed.
And remember, frequent breaks to walk around are essential.