Sit up with back straight and shoulders back. Buttocks touch back of chair. All three normal back curves should be present while sitting. A rolled-up towel can be used to help maintain the normal back curves. Keep arms bent at right angles at the elbow when using keyboard. Keep spine supported by keeping back against back of chair (If sitting on a sofa, use cushions to support back and laptop cushion so it doesn't slide around) Sit upright with elbows under shoulders. Avoid leaning forward.
Good posture. Reduce back pain.