I think it will take a lot of conscience effort at first but hopefully, like Tan said, after you have strengthened your core muscles it will become easier. Swimming can really help too. I am also trying to improve my posture so I make sure I check out my reflection in store windows etc where-ever I go and inevitably end up straightening myself up. You think you will look weird walking so straight at first but trust me it doesn't! Also flat shoes really help.
Good on you for trying to nip this problem in the bud!