Prep a toiletry kit + don't be afraid to pee - a lot.

The last time I flew to Europe I followed the lead of some thread I read from flight attendant and packed a toiletry kit for the plane with makeup wipes, perfume, toothbrush, paste, lip balm, and some make-up. About halfway through the flight I got up and spent 20 minutes in the bathroom washing my face and beautifying, and I landed feeling soooooooo much better. 10/10 recommend.

Also, sidenote - I'm chronically dehydrated, and it gets so bad when I travel. So drink plenty of fluids, and don't worry about getting up to pee. Just do it. The last flight I took was only 5.5 hours and I got up to pee three times. Worth it!