One of my many mantras is that everyone leans backwards— when they think they are standing up straight. A common trait of bad posture is a tucked pelvis that forces the legs forward and the upper body backwards creating the leaning backwards style. Fixing your bad posture is as simple as reversing this consciously and waiting for the muscles in the area to adapt to their new position.
If you think of the body as a hinge with the pelvis as the pin, the hinge should be fully open. We tend to lean backwards moving the pin and the legs forward. Take your thighs back and your upper body forward to start changing the way you stand and move.