Whenever I travel I pretty much stick to this 'uniform' because it has worked for me every time. You're truly dressing for comfort on long-haul flights so....From bottom to top: flip flops (occasionally tennis shoes for shorter flights), black sweatpants, black sleeveless top, pashmina scarf (you can find cheap ones on Amazon). If you're changing climates, I still suggest this wardrobe however bring jackets, change of shoes etc. I carry my large Kate Spade bag (I have the guava color ish, my bag is a little darker pink) and my school backpack and everything fits really well in these two.
(shirts available in other colors--all Forever 21)
Toiletries: Lotion, Eye Drops, Pimple Cream, Lip Balm, Toothbrush and Toothpaste, Medicine, Hairbrush, Tampons and Pads, Deodorant
Basic upkeep is really the rule of thumb on these flights. When you're in a tube of recycled air for 7+ hours your skin begins to dry out very quickly. Moisturizing lotion can help keep that at bay. Eye drops because along with your skin your eyes are sapped of much of their moisture as well. Personally I always break out after long flights so I bring pimple cream as a sort of pre-emptive strike against blemishes, and lip balm because your lips need some serious TLC during this time as well. Any medication that you need to take everyday should be brought on board with you (along with a few extra in case you get snowed in/delayed somewhere) as well as headache/cramp relievers, sleeping aids, and medicine for any other problems that may pop up.
Electronics: iPod, Headphones, Phone, Books, Magazines, Computer, Chargers
I pretty much don't go anywhere without my iPod and headphones and I need them especially on a long journey. They made a four hour layover feel like a 3 hour layover. I always bring both books and magazines, magazines because I don't really read them otherwise and it's nice to have a sort of fluff-piece to read, and books because once you're done with your magazines what else can you read? A 12 hour flight is a great time to get some reading done. I always bring all chargers for things in my carry-ons in my carry-on just in case I need to charge or, universe forbid, have a delay somewhere. Extra Tidbit: I always pack my DSLR camera in my carry-on, 1) because it won't truly fit in my checked luggage, 2) because I worry about it getting broken in checked luggage, and 3) because it can get stolen out of my checked luggage so best to have it with me.
Clothing/Comfort: Blanket, Thick Socks, Scarf, Jewelry/Expensive Things, Sunglasses, Change of Clothes, Neck Pillow, Eye Mask
I am constantly freezing on flights so I make sure to bring the fuzziest, warmest blanket I have with me as well as thick socks that I immediately change into once I'm seated. I wear a pashmina scarf around my neck throughout airports and once on board I wrap it around my bare arms and shoulders to keep them warm. Bring along sunglasses and anything you'll want immediately at your destination so you don't have to dig through a bulging checked bag. I normally bring my big bag of jewelry with me on the flight because checked luggage does, unfortunately, get robbed occasionally so I always suggest bringing things you absolutely couldn't replace or live without on board with you. As for extra clothes...at the very least bring a change of underwear and at the very most an extra outfit to change into upon arrival.
Snacks/First Aid/Misc: Hand Sanitizer, Wet Wipes, Snacks, Water, Luggage Locks/Luggage Straps/Luggage Tags
Always bring hand sanitizer and alcohol wipes with you! Germs run rampant on airplanes so use hand sanitizer often; I use it after every trip to the bathroom because even though I wash my hands, I have to touch the lock knob and door handle to get out. Airplane food is typically terrible so always pack a granola bar or two (make sure to check restrictions on some food items when you enter new countries, even if only for a layover) and even if the food is good, you're bound to get hungry. Water can't go through security so I buy a bottle near the gate and/or bring my own reusable bottle for the flight attendants to fill once on board. Hydration is key to avoiding dry skin, excessive bloating, and keeping you full to avoid munching on airport junk food. Finally, not really a carry-on but also an essential: lock your luggage! Avoiding checked bag theft can be as easy as getting TSA-approved locks for your luggage. Also, luggage straps and tags are easy ways to identify your bags on the carousel.
What are your carry-on essentials for a long-haul flight?