Try and relax on board, and consume only food that is easy to digest. Be sure to drink enough water, too; and go easy on the alcohol, coffee and tea. Alcohol will not necessarily help you sleep; but if you generally have a glass before you go to bed, keeping to the habit may help you do so. Coffee will tend to counteract fatigue, so only use it to deliberately influence your sleeping rhythm. Try to avoid drinking any coffee within two or three hours of any longer sleep you plan.
Avoid sleeping pills if you can: try to use autogenic training or relaxation exercises to get you to sleep instead. If you really want to take sleeping pills, take the lowest possible dose.
Try and keep your legs free by not putting carry-on baggage around your feet in front of you. Stretch your arms and legs regularly, and do the exercises suggested in the inflight magazine, too (raising and lowering your feet, making circling motions with your feet, and “rocking” on your feet from the heel to the ball and back).
Since the humidity of the air at cruising altitude is lower, i.e. the air is drier, your body will need to consume more fluids than it does on the ground. On long-haul flights in particular, the dry air can dry out your mucous membranes and lead to skin irritations, too.
This is why “drink more and eat less” is a good rule to follow on board – especially as your digestive system will be working less intensively anyway while you are aloft. The best things to stick to are low-fat foods, fruit and vegetables. And try to drink a glass of (preferably still) mineral water or fruit juice every hour.
A further drinking tip: since children have a greater skin area compared to their weight, and also tend to lose more body fluids through their skin than adults do, particular care should be taken to ensure that they drink adequate amounts of fluids during the flight.