Obsession is the way. I used to do the sticking out my butt thing Poloboy mentioned until I was made aware of it, and then I obsessed about it for a few months. My trick was to find things I did standing/walking every day, like riding the subway or wait in line at the cafeteria, and resolve to have perfect poise at those times. After a while it became part of my routine to check my posture at those times, then I started checking all the time and now I just have it. Pretending to be a noble lady of the court or a starfleet admiral helped, too!
You can pretend to be either a charming diplomat, a famous ballet dancer or a Prussian drill officer depending on mood. Have fun and think of how handsome you'll be!