My last flight (about 2 weeks ago) I wore:
Black cotton leggings
White babydoll top (also cotton)
Gray shawl-collar cardigan
Print
jamawar shawl (about as big as a blanket--love!)
black ankle boots
big *ss bag for all of my carry-on necessities (magazines, water, magazines, iPod, magazines...)
I think the most important thing is layering with breathable fabrics, as many people have said above. And be sure your shoes come off easily!