Good posture makes people look better. It makes you look younger and inspires confidence, and it feels good.
Good posture is a habit, and so is poor posture. The military is effective at training good posture. Typical causes of poor posture are psychological, lack of training, aging, and physical weakness.
If you want to improve your posture, just imagine your Mom reminding to to sit up, or stand up, straight, all day long. Then you can work on your abdominal and back muscles, which are what make standing upright possible. Here you go: Exercises for Better Posture.
Life training matters, and can only come from home. Moms say "Stand up straight, look people in the eye, have a firm handshake and a pleasant but reserved demeanor. Nobody wants to know your natural self."