So often people don't even realize how they're holding themselves. I do it myself, I hunch my shoulders over all the time and then I complain about how my shoulders hurt!
However if you purposely relax your shoulders you are making a positive change to your body.
Every so often when you're sat at your desk, watching telly, walking the dog, just take a second to think of your shoulders, let them fall down your back.
It will help your posture, keep at it and you'll notice the difference I promise!