The Tao of sitting at a workstation:

1. Sit with your back straight. Your head should float above your shoulders. If your vertebrae are all sitting on top of each other, you don't need to use your back muscles to keep yourself upright. Hence, don't lean on the back of your chair. If necessary, take up yoga or tai chi to free up your lower back, which through years of sitting badly is probably not able to keep itself in this position.
2. Position your monitor so that you can see any part of the screen without holding your head in an awkward position. When you tilt your head, make sure your neck is extended and bending, and not just collapsing on one side.
3. When typing, "float" your arms above the keyboard, do not put their weight on a rest of any kind. Ensure there is always a fist or so's gap between your arm and your torso.
4. Get up, and stretch when your body tells you to. Bend down and touch your toes (hinging your body at the waist, not bending your back!). Stretch your hamstrings and quadraceps. Extend your arms out, and pull your fingers back. Do a couple of Qi Gong stretches if you know them. Pop your knuckle joints with an expansive movement, to circulate the fluids.
5. Make sure you are fit and get enough excercise. If done properly, it is possible to keep a perfect physique with only a few hours no impact, no pain excercise each week.