The military is big on posture and they do a lot of marching with good posture. You could try going for a daily walk with the sole purpose of keeping your back straight, head up and shoulders back during it. Look at videos of soldiers doing this on YouTube. Imitate them. You don't want to do this to that extreme every day but doing it regularly will make you stand up straighter than you normally do, but not that extreme posture unless you mean to. Walking around like that will probably look odd, so depending on where you are you might want to do it at night. Or inside. Or if you don't care then do it anywhere. Or ask somebody you trust if it looks as ridiculous as it feels, because it feels ridiculous, but it ingrains a habit and that is what you want.
On a side note, in Victorian and Edwardian society in England the true aristocrats could easily be separated from the new money people and the poseurs by their posture. They had extremely good posture and sat very still for long periods of time at the dinner table without fidgeting or shifting and their backs not touching the back of the chair. They were taught this from childhood so it was habit and their muscles were used to it and it didn't bother them to do it. The folks who tried to imitate them hadn't been taught that nor the importance of it. The real aristos weren't going to clue them in, and while to me and you they wouldn't see very different, to those who were raised in that very picky society, it was obvious.
_________________
I'm giving it another shot. We will see.
My forum is still there and everyone is welcome to come join as well. There is a private women only subforum there if anyone is interested. Also, there is no CAPTCHA.
The link to the forum is
http://www.rightplanet.proboards.com