A well-adjusted chair reduces the strain on your back, and brings back the comfort of working in an environment where you have to sit in a chair for hours at a stretch.
Always notice the distance of the monitor and your sitting position. You need to be at least 12-30 inches away from your workstation to lessen the strain on your eyes. Use a chair that can be adjusted according to your height. You'll feel a little heightened, but it's worth it, because you don't put unnecessary pressure on your back. Also ensure that your knees and hips are in the same line.
In order to prevent back injury, you should be sitting up straight while at your desk. If your chair isn't providing enough back support, try using a rolled up towel or cushion until you find a position that's comfortable for you, and then adjust the chair accordingly.
Now that you've got your chair correctly positioned, take a look at your feet. Are they flat on the floor? If not, you may want to consider getting a footrest. This will relieve any pressure on your joints and muscles. It's important that you avoid crossing your legs or sitting with one (or both) legs twisted beneath you.
Actually there is a counter argument that says because your calf muscles are pointed towards the ground, you need to give them a rest and resting only one leg on the floor. To that effect, until recently most books prescribed this to be one of the many ideal positions. Well the logic is that when you sit on one leg, it's possible that you block the flow of blood in the other leg and therefore get it numb in no time. You could use a footrest but remember, it should be placed in a position that helps you feel comfortable and also places both your feet on the same level.
While being comfortable is about using the right chair, utmost comfort can arrive by simply taking short breaks from time to time. Moving your gaze away from the screen can also help you in more ways than one. After all, it is not your feet and sitting positions that need to be adjusted. If you feel your monitor is not placed at an accurate height, you could adjust that too. Sometimes, people place a small pillow at the back of the chair for additional comfort. Most high profile secretaries do this as they have to work for hours together.
Drinking water constantly also keep your cells fresh and replenished with water. While working in an office environment, make sure that you make yourself absolutely comfortable as possible, because comfort can ease you and bring out the best out of you in terms of productivity.