There are four things I would never get on a long-haul/international flight without:
1) Baby wipes: These can be used to wipe any sweat/smells away, give your skin a clean and refreshed feeling, and they just smell great!
2) Lipstick/Lip Stain: If you get a good, versatile shade of lipstick or a stain, it can be used for lip colour, blush and eyeshadow.
3) Disposable Toothbrush: These are those tooth brushes where toothpaste has been infused, or dried, on the bristles. All you do is wet them, and it's just like brushing with normal toothpaste. You can get them at dentists or pharmacies, I think.
4) Moisturiser: Hands down, I would never go anywhere without moisturiser. Hydrated skin is the best kind of skin.
Also, make sure to drink plenty of water and avoid caffeine or sugary drinks the day of, and before, you fly; they're quite dehydrating.