In regards to Diana always looking put together, i believe that it was her upbringing. A lady of her class would be taught from a very young age to always look clean neat and polished whenever she steps outside. People would always be looking at your appearance. Movie stars, Jpan Crawford, Bette Davis et all, always were dressed to the nines wheneveer they were outside-they had an image to uphold.
I guess thats an older generation ideal I guess. Even Audrey Hepburn, Grace Kelly, Liz Taylor always put together even when they went grocery shopping.