One of my pet peeves is people not wearing appropriate clothing -- whether being underdressed or overdressed. You don't wear your bathing suit to the supermarket and you don't wear a cocktail dress to a backyard barbecue.
However, I don't think I could have said it better that Erin does over at "A Dress A Day". She addresses it quite well in "What to Wear on Airplanes".
7 months ago