FLYING & STAYING HEALTHY
It is good to keep moving during a flight - this reduces the risk of conditions brought on by stasis, such as deep vein thrombosis, constipation, bad breath, swelling and varicose veins.
Ideally you should get up and move every 30 mins, although it's not always convenient and the person next to you might not appreciate it!
One of the most important things is to keep the calf pump mechanism going - so wiggle your toes and ankle to get the muscles activated and blood/lymph moving. You can do that in the seat. Even better if u can get up for a little walk down the isle. Children are generally on the go and wriggling around, but a fun way of keeping them active on a long-haul flight is to make them write their names with their toes.
Keep drinking water, and if possible avoid alcohol and caffeine as it will speed up the dehydration process.
Take a small pot of moisturiser with you to rehydrate your skin, particularly on your face and hands. Massaging it in to your skin, working from the extremities towards the heart, will also help with lymphatic drainage and blood flow.