The carry-on cabin luggage is always a compromise between what you might need or want during the flight, and what you are willing to carry and allowed to take on board. However, when flying long-haul, or even medium-haul (e.g. Europe to North America, with flights taking 6 to 8 hours), you need to consider carefully the essentials that will contribute to a less tiring and more pleasant journey.
Four absolute essentials
(1) Essential medicine. If there is any medication you have to take, pack it in the carry-on bag. Some types might need a special permission (e.g. insulin or anything else that requires syringes). Have a prescription with you as well, and enough supply to last you at least an extra 24 hours. Smokers should have a supply of nicotine gum or whatever else they are using to survive a flight, while everybody else would do well to have some painkillers, antacid, diarrhea remedy and some plasters (band aids).
(2) Money, cards, travel documents.
In addition to passports and paper tickets or printed out reservation details for ticket less flights, carry all your money with you on board. This is safer (no insurance will cover money or cards if they were checked in) and might come in handy if your luggage gets mislaid or delayed. Have insurance details with you in case you need to make a claim or authorize an expense (for example an overnight stay because a flight was canceled or delayed).
(3) Electronic valuables.
You need to carry on all your electronic valuables to prevent them from being stolen from the check in luggage (many insurance providers won't insure checked-in valuables). A laptop, camera (especially an SLR, cheap compacts might be risked in the hold), mobile phone, MP3 players and game consoles all need to come with you into the cabin. You will probably want to use them anyway for entertainment or information.
(4) Contact details, reservations, return tickets.
Any information relating to the onwards journey should be with you in the carry-on bag as you will need it even if the luggage is mislaid or delayed. Return tickets, hotel and car reservations or contact information for an organization or a host that you ill be staying with might be needed when going through immigration (which is always before the baggage collection).
Seven extras worth considering
(5) Change of clothes.
For a long-haul flight, a light change of clothes is a good idea, especially if traveling with children. If you don't have enough space, take at least a spare long-sleeved top.
(6) Pillow, blanket, eye-mask.
You should be given an eye-mask, a pillow and a blanket during most long-haul flights, but medium haul flights that are not nigh-time ones might not provide these. Many non-frill airlines don't provide pillows and blankets either. Check before packing.
(7) Flight socks or slippers.
If you have a tendency to swollen feet (or if you worry about DVT), bring flight socks or compression stockings. Otherwise, make sure you have big socks or slippers so you don't have to wear your shoes all the time or
Most long-haul flights will have some entertainment system and many medium-haul ones will too, although quality varies. Short-haul flights usually don't. Bring a book, a magazine, a crossword puzzle book, and load your electronics with audio books, music, games and even a movie or two, just in case. Bring a travel guide for your destination. If flying with no-frills airlines, don't forget earphones/headphones as otherwise you will have to pay for them.
(10) Basic wash and make up bag.
Especially if flying long haul, and with changes of planes or refueling stops, you might want to wash or even shower at some point. A small, absorbent towel, a change of underwear and a toiletries bag may make all the difference. Remember fluid restrictions still in effect (2011): take travel size toothpaste, small bottle of shower gel/shampoo and a small bottle or jar of moisturizer. If you can't live without make-up, bring some of that with you too. Men should have a disposable razor.
(11) Favorite snacks that might not be available.
Depending on the airline and class you are flying, you might want to bring some snacks that will cheer you up and relieve boredom or hunger. Chewing gum, candy, dried or fresh fruit are all in short supply on board planes.
When flying within Europe, often all your luggage will need be the carry-on if you want to save money on check-in bags' fees that many no-frills' airlines impose. When flying short-haul (and that includes many overseas flights within Europe or Asia), the carry-on is not crucial as the flights rarely take more than three hours. When flying long and medium haul, the choice of cabin luggage essentials can make a difference between a comfortable flight and one that is a frustrating nightmare.