[dkarl]

The only thing that has improved my posture (other than concentration and awareness, which are usually engaged with other things) is exercise. For me, running helps a little, but the best by far is lifting weights. After I lift weights, my posture takes care of itself for the next 12-24 hours. Lifting weights also helps me learn what a strong, healthy posture feels like, so when I consciously try to stand up straight, it's easier to achieve a relaxed, natural-feeling position instead of straining and feeling unsure.

I expect this advice only applies to exercises done on your feet with free weights, such as squatting and overhead pressing, not to bench pressing or machine work.