When I fly, I want it all - style, function, and comfort. And who says you can't have it all? I don't buy the idea that you have to wear sweats on a plane to be comfortable on a long flight. While I love my sweats at home, I don't think looking like you just rolled out of bed is cute when traveling, and numerous studies have shown you receive better service if you look respectable.
I know everyone is different, but I am super comfortable in skirts and tights (especially J.Crew's super opaque tights). Skirts are great when sitting for 9+ hours, while the tights keep you nice and cozy. For tops I opt for something looser, yet still flattering, like my favorite chambray shirt. In winter months I always wear a scarf (doubles as a blanket or shawl) and carry on my heaviest coat to save space and weight in my bag. And of course I carry my clutch-wallet, which is perfect for travel.
When I travel in the winter and spring months, I also usually wear my Hunter Tour Boots on the plane. These are very comfortable and light weight. They are packable, but wearing them still saves plenty of space in my bag. Finally for international flights, I never travel without a hat and sunglasses. These come in handy when you land and look a hot mess. You can read more about my luggage recommendations for long/international flights here, and my carry on packing checklist here.
What do you wear on a long flight?