Most of my flights are 10+ hours, so these are the measurements that I take
Before: stay hydrated and eat healthy for the last few days before the flight. Avoid alcohol if possible. Apply a good moisturizer (I swear by decleor's products, and I usually apply the hydra floral night cream before a flight)
During: personally I've found that avoiding carbonated drinks adds to my overall comfort during the flight. I make sure to stay hydrated, so I always grab a glass of water or juice when the cabin crew offer it, and I always bring at least one bottle of water/juice/lemonade.
I don't wear makeup during the flight, so I can apply moisturizer or face mist without making a mess. I usually use the night cream mentioned above, or the hydra floral moisturizing mask, and I usually apply it a few times over the course of the flight. 3-4 hours before landing I put on a face mask sheet. The ones I currently use are from skinfoods (a Korean brand) to freshen up.
After: I go to the airport bathroom to freshen up. I use blotting paper to get off some of the sheen from all the heavy moisturizers, apply eyeliner and brush my hair and teeth.