Tomorrow I'm leaving for Chicago, which is normally a 12-hour flight, but I was late and couldn't get a seat on the direct flight so I have to transfer in San Francisco, turning 12 hours to 17. 12 hours is rough enough.
Regardless, being comfortable on such a long flight is my main priority and I've naturally put a lot of thought into it. Denim leggings (I will never say the j-word) are perfect in maintaining a casual chic look while still being completely comfortable. A loose t-shirt and barely there bra are also key in this. Planes have a tendency of being ice cold, so I always bring a cardigan and/or big scarf along onboard (which can also easily turn into an extra pillow if needed). As for shoes, I usually opt for flats as long as they're cute, make me look good, and are easy to take on and off.
The bag, of course, needs to be big enough for all my personal entertainment and in-flight necessities, while still being awesome. I always bring a magazine, book, and some sort of handheld gaming device...though I've unfortunately lost my DS somewhere in my apartment. The air in planes is very dry, so on long flights I make sure to bring along some undereye cream as well as some kind of skin refresher (sprays work best for quick, mess-free application) to keep my skin moisturized. I always seem to break out a bit on planes, so one that promises a clear complexion doesn't hurt.
If you want to look fresh when you step off the jumbo jet, bring along some dry shampoo for a quick touch-up. Although an aerosol can admittedly wouldn't be ideal to try and bring through security.
I don't like wearing makeup for long periods of time whether I'm on a plane or not, so I take it easy. A swipe of mascara and lipstick, and a bit of blush (I'm so pale that I look like a corpse without a little color), and I'm good to go.