I know, I know! I have bad posture.
Sometimes I catch a picture of myself or a sideways glimpse in the mirror and think, "Wow, that's pretty slouchy."
I've tried to stand straight, etc. etc. The problem is that it isn't easy to remember to do these things all of the time. Also, many exercises and stretching aren't suited to being seated. And since I sit at my computer at work all day, it just adds to the whole mess.
But... I've found a quick "tip" that actually works. And it works seated. The trick is to turn your palms up. You'll instantly sit straight.
Go here to learn more.