The BEST exercise I know is to have someone coach you into good posture and then stay in good posture.

No kidding.

I've always been fascinated by posture and how people hold themselves, but my personal "ah HAH!" moment was when I was working with a Pilates instructor to design a bridge class for back patients who were transitioning from PT to Pilates. She taught me things about my own posture that were amazing.

If you can get a really good Pilates instructor for some one-on-one (or an Alexander Technique instructor, or a Yoga instructor, boxing coach, dance teacher, whomever) and have them go over your posture, it is so valuable. Those outside eyes see the things you can't feel, and can coach you to arrange your body into subtle corrections that you never knew were there.

If they give you a blank look and say "well, I can give you some exercises" then they are probably not high caliber. Someone who really knows their chosen discipline (whatever it is) will be able to create hours of work and sweat and tears for you out of just standing still.

And that's a core workout!