How can I consciously work on better posture; if im sitting in the couch a lot my back and arms start hurting if im up and about I feel better.

Small pillow. One way is to place a small pillow or rolled up towel across the bottom of your back when sitting in a couch or chair. . Push back against the pillow/towel and it will bring your shoulders back to prevent slouching. Eventually your body will become accustomed to this position.... Better posture!