This was a few years ago, but I saw some tricks to good posture on Oprah. Let's see if I can describe them, without making it sound too silly.
While standing, exhale hard. Feel where your belly button is? Leave it there. What this does is tucks your hips under the body. My dance coach also tells me to push my buttocks down, which was the best thing ever. Before I tried to tuck my hips in my squeezing the stomach, which unfortunately also squeezed the bladder. Not a comfortable feeling while dancing.
So that was the hips. For the shoulders, it was suggested on Oprah to let the arms hang loosely at the side, palms facing the thighs. Then rotating at the elbow only, bring the forearms up so that the arm is at a 90 degree bend and the palms facing up. This should cause the shoulders to rotate back and down. Which while doing this, I realize that if you bring the arms up from that position into dance frame, you will be doing what my dance coach calls "giving your partner your elbows." Which supplies a nice frame and back.
Lastly, the abs play a good part in posture. A Prevention magazine article one time stated that one of the best exercises for posture is walking backwards (Golly gee-whiz!!) This causes the abs to work to bring the body in line.
Dancing has helped my posture immensely, it's taken time, and I'm not completely there, but that's more mental. Until I can break this mental wall of wanting to fade into the background, it probably won't happen. But the trick to good posture is practice, practice, practice. The best feeling I ever had was walking down the street realizing that my body was automatically "sucking the ole stomach in."
GoodLuck
train your muscles of the back by yourself in front of a mirror.. thats work but nobody said dancing is always fun.