Flying has become such a routine part of modern life that passengers now usually wear comfortable clothes. I recommend loose-fitting clothes and shoes that are easy to kick off.
If you can't relax without your own pillow or blanket, you can take it with you. If the flight is a long one, it is best to focus on getting a good rest.
Warm clothing is a good idea even if you are headed for warmer climes. The air-conditioning is always on in the cabin, and temperatures are set rather cool for many reasons – to prevent any medical crises, for example.
Earplugs and sleep shades may come in handy too, and some people never go anywhere without a trusty neck pillow. Eyeglasses may feel more comfortable than contacts, as the cabin air is very dry. Dry air is hard on the entire body, so drink as much water as you can.
Alcohol is not such a good idea, though: it dries you out even more and has a stronger effect during a flight than on the ground.
You can moisturise your skin properly before flying and maybe even carry a spray moisturiser. Many flight attendants find these helpful. After a long time sitting down, your body will thank you for a little stretching and flexing. Rolling your ankles and wiggling your toes doesn't require much space.
I also recommend compression stockings or socks, another handy item for many flight attendants.
Don't go hungry when flying
Many ask for exit-row seats, but remember that passengers in those seats must be able to help the flight crew if an emergency arises.
Families with small children are often seated in the front section because in large aeroplanes baby seats tend to go in the first row.
Long flights include a meal, but I still wouldn't board a plane hungry. You might not get to eat at all if, for example, a last-minute delay puts you on a different flight and any special meals ordered don't allways make the new flight.
Baby food is allowed in the cabin despite the current strict rules about carrying liquids. The crew can warm the food for you, but it probably won't be very quick: they do this by placing the bottle or jar in hot water. At times, we get asked for special 'flight attendant tips' for longer flights, but a few simple things go a long way: enough water and a little exercise.
One specific tip I could give women is to wear eye make-up primer; it can keep your look tidy even if you inadvertently rub tired, dry eyes.
Have a pleasant flight!