Avoid alcohol and caffeinated drinks during the flight as they will lead to dehydration and tiredness.

Avoid eating a heavy meal during and before the flight as this can lead to an upset stomach given your relative immobility during the journey.  Try to consume light snacks like a fresh salad instead.

Keep moving throughout the flight, get up and walk about for 5 –10 minutes every hour.  Stretch regularly in your seat and avoid sitting with crossed legs. 

Tense and relax you leg muscles and place them on the foot rest as often as possible.  Jiggle your legs in a seat position, and try doing a peddling motion – anything to keep your legs moving throughout the flight.  Immobility can lead to DVT.

Try pressing the balls of you feet hard down against the floor or foot-rest, as this can also help increase the blood flow in your legs, and therein reduce blood clotting which leads to DVT.

Wear elastic in-flight socks which can prevent DVT.

When taking off and landing, try sucking sweets or swallowing.  This will help equalize the pressure in your sinuses and prevent ear and sinus pain.

Always carry all medication as hand luggage and have to hand in the case of an emergency. 

It is a good idea to carry a document outlining any allergies or special medical conditions on your person, in the event of an emergency.  Remember, not everyone reads English, so if say you are flying from Mexico to Argentina, it is worth getting a translated version in Spanish.

Although very rare, British Airways estimate that one in 20,000 travellers experience a serious health problem whilst flying.