When I was about 11 I read that sitting/standing up straight affords one more energy and thus I adopted the habit. Then 2-3 years ago I made a point of leaning back in chairs more just to seem more human so people could relate to me better. Before, I never touched the back of chairs as I didn't lean back at all. At least I got plenty of compliments on my posture
Making myself slouch made my back sore for a while but now it doesn't feel like so much of an issue.
Sitting up straight still feels easier than slouching though. One thing I've noticed is that a lot of chairs (particularly easy chairs/couches) are designed to make you lean back, and that you'd actually have an easier time of sitting up straight in them if you did a 180, with your chest facing the back of the chair, or simply sit on the edge of the chair, which would make you seem more interested in whatever was going on anyhow.