A really great system for posture is called the Alexander Technique. It’s used a lot by dancers, actors, musicians, etc. It works by changing the way you experience your body from the inside out, so to speak. It requires a lot of mental work, but not a lot of physical sweat. Google “Alexander Technique” and you’ll probably find something useful. Or ask the librarian at your local (secular) library.
I had terrible posture until I took up martial arts, but I don’t think too many frum girls in Bklyn would be open to that. The Alexander Technique is a little less drastic, and even a bit of fun.