A few years ago, I got rear-ended in my car, and suffered back injury. To treat the pain, I got physical therapy. One of the methods to improve and prevent back pain, was to improve my posture. My therapist told me that it put a lot of stress on the back to slouch.
It took a lot of practice to improve my posture. My muscles and joints were just so used to slouching. But that wasn’t the only thing. It was a cultural/social issue too. Just about everyone slouches, so when I sat up straight, I felt out of place, like I was being too snooty or something.
To reinforce better posture and prevent slouching, my PT would gently poke me in the back when I slouched. Eventually my posture training took hold and it became easier–both physically and mentally–for me to sit up straight consistently.
A few months ago, I had a physical therapist; this time it was for my shoulder rehabilitation. He praised my good posture. It was a great compliment coming from him. It was like being complimented on an outfit by a fashion designer.
Now, I am so used to sitting up straight, I would have to make a conscious effort to slouch, which now feels physically uncomfortable to do.