Sit up with your back straight and your shoulders back. Your buttocks should touch the back of your chair.
All three normal back curves should be present while sitting. A small, rolled-up towel or a lumbar roll can be used to help you maintain the normal curves in your back.
Here's how to find a good sitting position when you're not using a back support or lumbar roll:
Sit at the end of your chair and slouch completely.
Draw yourself up and accentuate the curve of your back as far as possible. Hold for a few seconds.
Release the position slightly. This is a good sitting posture.
Distribute your body weight evenly on both hips.
Bend your knees at a right angle. Keep your knees even with or slightly higher than your hips. (use a foot rest or stool if necessary). Your legs should not be crossed.
Keep your feet flat on the floor.
Try to avoid sitting in the same position for more than 30 minutes.
At work, adjust your chair height and work station so you can sit up close to your work and tilt it up at you. Rest your elbows and arms on your chair or desk, keeping your shoulders relaxed.
When sitting in a chair that rolls and pivots, don't twist at the waist while sitting. Instead, turn your whole body.
When standing up from the sitting position, move to the front of the seat of your chair. Stand up by straightening your legs. Avoid bending forward at your waist.