How do I not be so hunchback??

I have a pretty bad slouch and its getting worse. I drink at least 1 or 2 bottles of water a day (not much soft drinks) and thats all I know to do. Is there a certain exercise I can do to not be so slouchy? I try to stand up straight, but then I just get back to normal hunch after a while. Also, at the end of the day, my back really hurts and I think its because of my heavy bookbag. How can I stop looking like Quaisimodo the hunchback?