watch posture whenever possible until its a habit and pushups/situps/handstands to build strength so its less tiring and more comfortable should do the trick
I used to have AWFUL posture. Running made all the muscles stronger needed for correct posture. So I suggest running or walking while maintaining the proper stance.
Pilates!! Seriously. I've taking classes with a private instructor and I always walk out with noticeably better posture. It has to be done consistently though, at least 3 times a week. Better 5 or 6 times if you can (just like any posture-improving exercises)