I remember when I was a kid, before I developed the massive chest weight, my mother used to pick at me all the time because she said I was "standing like the letter S." I never was sure what that meant, but I think it referrred to some kind of slouching. She would tell me to imagine a string at the top of my head, pulling me up to the ceiling but I didn't like that thought because it felt uncomfortable.
I can focus on having good posture for about five minutes before it really starts to hurt. I would keep on through the pain (because I figure it's weak muscles complaining that need to be strengthened) but I can only focus on my posture -- I can't keep good posture AND do something else at the same time. So as soon as I'm in lecture, taking notes, on the internet, watching TV, or pretty much doing anything other than focusing on maintaining good posture, the slouch comes right back.
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"In the end, we decide if we're remembered for what happened to us or for what we did with it."
-- Randy K. Milholland
Avatar=WWI propaganda poster promoting victory gardens.