oh man. i have a forty eight hour flight in a couple of weeks. i say some soft denim (if you can find one, paper denim & cloth is a good brand) and a white tshirt, with layers, like a cardigan and a light jacket. this would be if you still wanted to look decent.
ill probably go in my AA genie sweatpants, or thick black leggings, soft t shirt, and hoodie, and my anorak. im trying to find a soft leather black backpack to wear! i really want the alexander wang one, but thats sooo out of my price range!
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!
I hate to say it but UGGs (for the extremely cold plane rides) and sweatpants and tees are the best way to go.
The last time I went on a plane I wore a short-sleeved dress and Christian Louboutin wedges, my feet were blue and my arms were freezing.
I am always freezing on planes! They have the air-con up so high, but that makes it all the better to get the whoosh of hot air as you step off the plane [if you're going somewhere warm obviously...]
Shop windows, sunglasses, spoons....Some days there just aren't enough reflective surfaces.