How can I improve my sitting posture?

It would help you to do a specific stretch as well as two exercises in order to make it easier on your muscles to keep your chest up while decreasing pain in your low back. You can stretch your upper body by standing at a corner and putting one hand on each wall at shoulder height. Lean forward until you feel a stretch in the front of your shoulders and hold this for about 15 seconds. Try to do this 3 times a day. You can strengthen your lower back muscles to support your chest by lying on the floor on your stomach. Keeping your belly on on the floor and your ankles close together, lift your legs off of the floor and hold them up for 10 seconds. Also try to do this 3 times a day. These should help but it will take a week or two for you to notice a difference so don't get discouraged!