A big component of your appearance isn't only superficial.
Proper posture and composure are critical to improving both your character, and people's perception of you.
The best part of practicing good posture is that it costs nothing, and you can accomplish it this... very... second.
This isn't anything you haven't already been harassed by before; if you're sitting, keep your chair in a upright position, with your back resting straight along the chair's arch. If you're often on your feet, train yourself to spread your weight evenly between both legs as you stand comfortably erect.
Either that or do a web search, or talk to your doctor or something. I'm just a tech guy.