Always try to wear comfortable clothes for a long flight. Remember that the plane seats will not get bigger, even if you are going to travel for a long time. Consider wearing clothes that you feel you will be comfortable in while sleeping. If you wouldn't wear it to bed, you shouldn't wear it during a long flight. Don't worry about fashion; you will have plenty of time to look good once you get there.
Be sure to wear shoes that can be removed and put on without using your hands. Being able to take your shoes off easily is worth its weight in gold while on a long flight. If the passenger in front of you reclines their seat, it's unlikely that you will be able to reach your shoe laces.
Bring your own earphones. Not only do airlines charge you for earphones, they are usually of the poorest possible quality. Good noise cancelling earphones will not only help you block out the sounds of your neighbours, but will also allow you to enjoy the in flight entertainment. Watching a movie that you cannot hear can be incredibly frustrating. Bringing along an MP3 player is an added bonus.
Be sure to pick up an airline earphone plug adapter before you go. Look for one that splits a common 1/8" phone plug into two separate 1/8" mono adapters. This will allow you to adapt your fancy earphones to almost any airline seat.
Bring your own snacks. Let's face it, airline food isn't exactly gourmet. Bring some healthy snacks to nibble on as you travel. Not only will it taste better, it will allow you to maintain your blood sugar as you travel. It's unlikely that your normal meal schedule is anywhere close to the dinner schedule of the airline. Avoid large amounts of caffeine and sugar to help prevent any sudden highs and lows. Most importantly, do not drink alcohol while in flight. The dry air in planes dehydrates you and alcohol only makes this worse. A few drinks may seem like a good idea before takeoff, but will come back to haunt you six hours into a transatlantic flight. Remember that going to the bathroom is sometimes a real adventure while in flight.
Have some hand sanitizer handy. With all the recent cuts airlines have made recently, planes are commonly very dirty places. It's hard to relax if you are constantly thinking about what the blob on the seat in front of you is. A quick application of hand sanitizer will help you feel fresh, clean and relaxed. Not to mention, it will also help you avoid the possibility of falling sick during your vacation.
Have your own neck pillow. Most of us don't sleep in a restrictive chair on a regular basis. Because of this, airplanes present a unique challenge for the sleepy traveller. A good C shaped neck pillow will allow the non-experienced and experienced traveller alike to catch a few Zs without getting a crooked neck.
Give yourself a schedule and stick to it. Take into account the time zone you are flying into. Think about your normal meal times. Give yourself a schedule that is half way between your home and your destination. If you decide to "power through" the flight, you will find yourself restless and groggy the whole flight.
Move around. Excuse yourself, get into the aisle and stretch your legs. Do some squats, arm circles and neck rolls. The more you get your blood flowing, the easier it will seem to be confined to a seat for 5-12 hours. Don't be shy; remember everyone on the flight is dealing with the same discomfort, so there is nothing wrong with asking to have access to the aisle to move about every couple of hours. Besides, your legs will thank you for it.