We all learned to sit at a young age, but bad habits creep in very easily, and the way you are sitting could be contributing to Repetitive Strain Injury(RSI).
There’s no one way to sit correctly, as we are all different shapes and sizes. However, there are guidelines that can help you determine if you are, and how you can fix it if you aren’t.
First thing is the height. Start by checking your height so that your arms are level when using the keyboard. Hopefully you’ve adjusted the monitor height as recommended by the earlier post. Keep in mind that you want your feet sitting level, so if they’re not touching the floor, try resting your feet on a phone book or foot rest. Gas lift chairs are a good option as they are adjustable.
Choosing your chair should be done with some care. Are you leaning slightly back? Do you feel each part of your back is properly supported. Check that the support is not just for the lower back, and that it’s not too much. Generally speaking you want the back rest to come up to your shoulders
If finding a chair is proving difficult, there are companies that make custom chairs, so you can get all the support you deserve.