Growing up I received my fair share of nagging about my posture, by being constantly interrupted to be reminded to stay straight.
Regardless of that I still developed really bad posture and health complications related to it. The effort of my parent's was completely innefective.
I'm sure that their intentions were good and they ment good, but the lack of understanding about how the body works, cause them to give innefective and useless advice.
It was like they were asking me to do something impossible, and to a large extent they were. Sit down and keep your back straight! It is an impossible request to comply to.
Sitting on a regular chair will unavoidably cause you to roll your pelvis back and lean forward. It is not the fault of the person that sits, but the fault of the furniture that encourages and facilitates the behaviour.
Also telling someone with bad posture to stand straight is an impossible request. The muscles that control your posture are not muscles that you have voluntary control over like the muscles in your arm. They are more like the muscles in your heart, completely wired to a different part of your nervous system.
So if you look at what someone does after is being cautioned about their posture, they will get really really tight, overcompensates by arching the back to lift their head and shoulders, and of course since this is an incredibly uncomfortable position, a few seconds later they slip back into the bad posture.
Not to mention what reminding someone that there is something wrong with them and that you are bothered by it does to someone psychologically, it destroys someone's self esteem. On top of that no matter how hard they try to self correct they will never succed because they are being asked to do something impossible.
Nagging people about their posture won't save anyone's back, it is just some kind of psychological torture, that nobody should ever go through.
It kind of all goes back to this idea that your posture is something that you have councious control, and that you can control by being reminded of it. You can look at some of the products on the market that are supposed to help with this problem, that are based on this idea.
Like apps that you install on your smartphone that use the accelerometer to assess your posture and send you reminders if you are not staying straight.
Maintaining a healthy spine or rehabilitating a sick one happens by changing your lifestyle. Like changing your environment or educating yourself. By addressing the root cause of your problem.
In the case of posture there are two fundamental changes that need to happen: one the prolonged sitting has to stop and replaced by other forms of movements, two the muscular imbalances that might have been developed in the past do to sitting need to be corrected.