TIPS TO IMPROVE POSTURE
Sitting for long periods of time is really terrible for the body. Get up and move every so often.
Uncross those legs. The correct way to sit in a chair requires you to place your feet flat in the ground.
Pull back your shoulder blades. Drawing them back and down will bring your chest up and out, and help you avoid getting rounded shoulders.
Try using a lumbar support pillow. A lumber support pillow does some of the work for you by supporting your spine.
Practice small shoulder and chest exercise.
Keep your head leveled. Your earlobes should be in line with your shoulders. Do not push your head forward, backward, or to the side.