I feel that being erect and well-postured is no different from having good materials used in constructing a building.
When I finished a year of taking therapeutic magnesium, with the intent of beefing up my magnesium stores for good bowel movement without relying on eating fiber (which I had always relied on pre-Peat), and which isn't good as fiber breeds bacteria that produces serotonin for bowel movement - I noticed it seemed natural and effortless for me to stand straight (like the way Ronald Reagan would stand, for I admired his bearing).
Together with having good calcium sources in what I eat, as well as magnesium sources, especially in well cooked leaves, I found that the body would more easily support itself .
Weston Price would observe the good structure of tribal people as well. They have a natural good posture, and they don't go to fashion modeling school to walk like models would.
I had bad posture growing up. And by some twist of fate, I met a car accident and I was laid to recover in a hospital for a month, with a minor fracture at my thoracic. I was put on a cast for 3 months after being released, and was told I have to do a daily backstretch exercise to develop back muscles to further give support to the spine, weakened by the minor fracture. All these I went through and did, and my posture improved. But I don't wish a car accident on anyone, and I recommend good nutrition instead, especially with calcium and magnesium. I'll throw in the other electrolytes for good measure.