Drink lots of water, and avoid over-eating, alcohol and caffeine.
Use moisturizer, lip balm and eye drops to avoid dryness caused by cabin humidity.
Keep your seat and leg area free of clutter. Stow your one-piece cabin bag in the overhead compartment at all times.
Regularly shift your body position while seated. Avoid keeping your legs crossed or sleeping in a cramped position for long periods.
To avoid jet lag, try to stay awake onboard if your flight arrives in the evening at your destination, and get some sleep onboard if your flight arrives in the daylight.