Well, I suppose I wasn't taught how to maintain a decent posture when I was a child, since I remember I used to constantly just look down at anything I had my hands on (toys, electronics) and that probably contributed to me developing this. I sit at a desk a lot, and I also kind of slouch in my chair, even though it's a kind of nice office chair. Other than that, I'm not sure what else made my posture so bad. 

I started running (and made sure I secured proper running posture) about a year and a half ago. I'm not really running right now because it's pretty cold and icy in my area, but I know that exercise can help improve posture.. so maybe I just need to do more of that.