I have good posture. I sit up straight naturally, even if I'm tired. It's just a habit.
I have scoliosis, which is curviture of the spine, and basically mine is *almost* curved like an 'S', but not soo curvy, LOL. (Although, you couldn't tell if I had it just by looking at me.) It's not a severe case, but I don't want to take chances, so over time I guess I just taught myself to sit up straight.