I completely agree with Adam. I've always had pretty good posture, but when I started with my personal trainer, he pointed out that my shoulders naturally came forward a bit more than they should and my knees pointed inward a little too much. I really had to think about bringing my shoulders back, and it felt really uncomfortable and unnatural.
Desk jobs result in underactive and shortened muscles. We worked on strengthening my quads, glutes, lats, and back. As my muscles became more balanced, my regular stretching became significantly more effective.
My posture is significantly better without me thinking about it.
Forcing your body into a "good posture" is not a good long-term solution, and, as Adam said, will probably exacerbate the underlying issues.