We all want it it seems. Not to slump or have round shoulders. To be able to stand up "straight". Interestingly people used to say to me in my pre-Alexander days 'you have great posture'. From the outside I looked pretty straight I guess but I was held and stiff. My breathing was shallow and I was uncomfortable sitting or standing for longer than 5 mins.
This is me sitting now, right at this moment. I am using The Alexander Technique to allow my head to be easily balanced on my spine. allowing myself to be supported by the chair and thinking of not interfering with my natural coordination. Its not perfect. There will be more and more of me that can let go of old habits. But its way better than my old rigid self. I can breathe more freely, move more freely and sit or stand for long periods easily and it feels so much better. I feel so much better.
So forget perfect posture, it usually involves way too much efforting. Instead learn how to find ease, balance and poise in whatever you do. Learn posture using The Alexander Technique.