Standing up straight seems pretty obvious, but I used to slump when I was a teenager. Proselytiser has a good point about keeping you chin parallel to the ground, I used to let my head slump forward. I had a friend who was very good with women. His mother always used to tell me to stand up straight. Looking back, it's no wonder he was good with girls, he was probably getting good advice on how to present himself right from the start.
My parents were great in other ways, but they didn't try to correct my posture. Or if they did, I didn't listen to them. Things like that can sometimes carry more weight coming from someone outside the family.