I slouch a lot. But I try to correct my posture whenever I can. I never thought I was doing a very good job of it, but in the past couple of months two people have commented that I have good posture so I guess it's working.
It's hardest to do when I'm sitting in an office chair or at a desk, my home and work desks both suck, basically can't sit straight for more than 4 minutes in either. Also on couches. I can sit up straight on a hard seat, like on the subway or rigid chairs at restaurants.
EDIT: I've been making an effort to keep good posture for probably a year now. Only lately am I finally feeling comfortable standing up totally straight. It's a huge difference; I feel taller when I do it too.