Hi ringingthechanges - I find this interesting because although have noticed her posture change, you also comment on her shyness.
There is such a strong relationship between physical demeanor and mental/emotional health that I wonder whether you dd's slouching is an indicator of a deeper issue?
If, for any reason she is feeling a bit depressed or unhappy, she will be holding her body in a way that represents that. Asking her to change her posture is probably as uncomfortable as asking her to go out on stage and sing to an audience.
My dd at age 14 tied her hair back into a horrible tight scrunchy bun - because she hated how she felt about herself with long flowing curling hair. She was at times depressed and she discovered for herself that if she 'skipped like a 4 year old' she couldn't feel unhappy.
As a complete alternative: dd2 (who is happy, confident, outgoing, party-goer) has scoliosis of her spine. This causes some bending and is very, very, very, common in teenage girls (google it). This can cause lop-sided walking etc. It tends not to be treated (unless it is so extreme that it compresses the lungs and causes breathlessness) and in any case treatment is usually a back brace.
If you have a rather shy daughter, then the best you can do is whatever you can to help her feel better about herself, her decisions and her abilities in life. If her posture is caused by "laziness" then she will change it as soon as there is an external reason for her to do so (potential boyfriend, peer pressure, sporting competition etc).
If her poor posture is caused by a physical problem such as mild scoliosis then, you would be best not to draw her attention to it.
If her poor posture is caused by her mental/emotional feelings then it is likely to be a tender walk for all of you to help her to stand up tall.
And of course the reality of teenagers is that it is likely to be all of the above, or none of the above. All we can do is love them.