Here is a recommendation from medical professional who authored the article found in the link below...
Step 1: Sit straight up.
Step 2: Draw your abdominals in so that your lower back is flat.
Step 3: Pull your rounded shoulders back by slightly pinching your shoulder blades together. Be sure not to shrug up, as this is a very common substitution. One of the best cues is if you draw the top of your shirt up.
Step 4: Perform a chin tuck.
Step 5: Inhale through your diaphragm and out. Do not allow your chest to rise more than your belly.
Those are the simple steps to consider! To counter some of the side effects from sitting for extended periods of time, the recommendation is to try these steps one to three times per day (under the guidance of your doctor).