Bring an inflatable neck pillow so you can sleep more comfortably. Use the airline pillow to support your lower back. Take an airline blanket upon boarding in case you get cold later in the flight. It can also be used for back support or as a pillow.
Use earplugs and an eye mask to block distracting noise and light.
Wear elastic-waist pants, plus a light weight T-shirt and a sweater. Wear shoes to accommodate feet that will swell during the long flight. Change into a pair of thin socks during the trip.
Drink at least 1 liter of water for every two hours of flight time.
Combat dry cabin air by misting the face, using moisturizing eye drops, lip balm and skin moisturizer (this goes for men, too).
Walk around every so often to stretch your legs.
Bring along a small bottle of hand sanitizer to clean hands before meals.
Pack a toothbrush in your hand luggage. Brushing your teeth after a meal during a long flight makes life a bit easier. And you feel a lot fresher when you arrive and are met at the airport. -- U. Brinkmann
Most airlines publish in-flight exercises you can do sitting in your seat to counteract the thrombosis syndrome. Look in your airline's In-Flight magazine where you can usually find these most beneficial exercises. I noticed British Airways is now doing this - on a recent flight to London/Dublin. -- Sharon O.