It is very important for long days at the computer, for work or for private use that you have a good sitting posture to avoid complaints. There are a number of rules that can help you with this. In addition, make sure that you vary your posture regularly, or go for a walk in between work.

1. correct seat height

Make sure your chair is high enough for your height. It is important that your feet can easily reach the ground. However, the knees should not rise too far, because then the chair is too low. The best height is that height at which your legs are at an angle of 90 degrees.

2. correct seating angle

Sit up straight in your chair with your back and legs at a 90 degree angle. Slide back on your chair as far as possible and lean against the backrest (make sure your backrest is at a 90 degree angle to the seat!).

3. correct table and armrest height

The armrest must be at the correct height so that it effectively supports the arms. While typing at your computer, the arm should be at a 90-degree angle. So make sure that the armrest fits exactly under the arms in this position. This prevents you from having to tighten your arms to type properly. The table top will then have to be at exactly the same height, so that you do not have to bend too much forward or the arms do not have to be lifted too much.
Source: http://www.rsi-alert.nl/werkouding.htm.php