You might want to look into this: http://www.lumobodytech.com/lumo-lift/
I haven't tried one myself, although I am tempted as I often get quite bad back pain (at age 23 :-/ ), however I can't quite bring myself to add yet another device to my life that needs looking after, charging, etc.
However, I wonder if you can solve this problem purely with software. Could an app detect slouching by monitoring the user through the webcam?