First, I think it is necessary to understand that poor, hunched or closed posture is a deeply subconscious attempt to introvert and close oneself. It is an effort to literally protect the vulnerable front of one's body - that is, the area where our vital organs (groin, gut and neck) are most exposed. Poor posture is a manifestation of insecurity, whether it be physical, social or emotional. The deeply entrenched nature of a person's posture makes it incredibly difficult - indeed, virtually impossible - to change by repeated acknowledgement and correction. I know people who have have been doing this for years, but their posture remains unaffected. I used to have poor posture myself, and for a while made similar "acknowledge-and-correct" efforts, but they were futile.
My posture now is very good. The change was not one I consciously forced, however. Instead, it followed naturally from an improvement in my self-image and confidence. This was a gradual process, but it was was marked by distinct points at which I recognized my value, and more importantly, my potential. This would happened when I saw some similarity between myself and someone I admired, or upon some concrete accomplishment that made me realize that I was capable of things I hadn't thought I could do (or do well) previously. Accomplishment breeds confidence, and accomplishment is the product of initiative and effort - both of which are controllable.
But my personal experience is not the only evidence for my point, by any means. Consider, too, the people you know who have the best and worst posture (aside from the elderly, who have less control over these things). Almost without fail, the ones that slouch are also insecure or very humble, and the ones that stand up straight do so in proportion to their confidence. The correlation is unmistakable.
So rather than straightening up when you recognize your poor posture, you would do better to acknowledge your strengths and excuse your weaknesses, at which point your posture will improve naturally. Far more importantly, if you want to stand up straight, you need to begin to do the kinds of things that will allow you to respect yourself. If you are too timid to carry on a conversation with a man, start practicing so that you grow more comfortable. If you hate the fact that you are overweight, start setting weight-loss goals for yourself and penalties if you fail to meet them. If you have been wallowing in the wake of a bad break-up, shake it off, get back on your feet and start dating again.
These things are not easy to do, but I've seen the effect they can have on a girl's life, and they are worth the effort. They also become easier the more you do them and recognize their benefits. Taking charge of the things that you've previously been unable to master is the best and possibly the only way to improve your self-image, and therefore your posture.