I believe that comfort is key on long flights, but sloppy is out. I might be comfortable at home wearing sweats but I'm not going to wear them on a plane. Usually I will wear long pants, as I tend to get cold when flying, and a nice blouse or sweater. You also need to take into account the climate you are going to. I've flown to the Bahamas in the winter and wanted to be comfortable when I landed and wore a light weight dress and hose (to keep my legs warm on the plane).

Common sense is all it takes and never wear anything that would embarrass your mother