Here's what you can do if you already exercise and have a strong back. Take some surgical (medical) tape, stand up straight with good posture and have someone run a strip of tape down your spine on your bare skin. This will produce a tugging sensation whenever you begin to slouch.
I have never tried this, but saw it in a magazine somewhere years ago and it stuck in my mind because I slouch also. I am exercising now and my back is stronger, so I may try this in the fall (I think my skin is too damp and oily in the summer for the tape to last
).
Good luck!