few things u cud do :Simply stand up straight. Keep your head up. You'll look more confident and you'll feel good about yourself. Just as your mother probably used to say when you were a child, don't slouch. She was actually right.Once you're aware of your bad posture, it'll help you get in the practice of knowing when you're slouching and reminding yourself to stop. When you are sitting down,make sure your back is pressed against the back of the chair with your feet flat against the floor.It's easy to slouch at a desk when surfing the internet. Using an exercise ball can help force you to sit up straight since you have nothing to slouch into. Best wishes
Other than just remembering to stand up straight, and get used to it so that it becomes a habit... you could try wearing undergarments that are made to correct your posture, as well as slim and smooth out your body.
http://www.funadvice.com/q/fixpostureguess The "best answer" really is the best one I've ever read having to do with posture.