Think string. Always imagine that a string coming from the top of your head is pulling you gently up towards the ceiling. Visualization techniques like this one can guide your sense of proper position and height effectively.
Have someone tape a giant X on your back from one shoulder to the opposite hip. Then put a straight line of tape across your shoulders closing the top of the X. Wear this during the day, to help retrain your back. This works really well if you hold shoulders back before taping, use wide non stretch tape and ideally change tape each day.
Avoid the slouch when walking. Be sure to try to walk as if you had a book balancing on your head
Use color. If you need help remembering to keep your posture, think of a unique object or color. Every time you think of that object, check your posture.