Some airlines now offer seatback entertainment consoles, offering movies, music, tv shows, and information on the current location of the aircraft.(seen here in a Delta Airlines Boeing 767 between Atlanta and Paris)
In cases of an open flight in economy class when nobody is beside you, feel free to put-up the armrests (except in exit rows where the armrests can't be lifted) to claim the extra space to yourself.
Wear your seat belt at all times while seated. More people are injured (a few even killed) by failing to do so than from all other causes of flying injuries. Severe air turbulence can occur without warning even in clear air, and can violently throw you and others about.
If you are using a coat/jacket or blanket to stay warm, fasten your seat belt on the outside so the cabin stewards can see that you're using it without disturbing you.
Seat Courtesies: Especially when others are sleeping or eating...
If you want to lower your seat back, you'll be taking space of any person behind you; so check first, e.g., if they're eating it could otherwise create a mess.
When getting up from other than the aisle seat, ask seat mates to let you out, and try not to disturb people behind or in front of you.
Drink lots of water, and avoid caffeine and alcohol. Otherwise you'll dehydrate at a faster pace, which can worsen jet lag and may induce headaches. Don't hesitate to ask the cabin crew for more water, or to walk up to the galley to request more. If sanitation is unknown, ask for bottled water if available.
Don't sit completely still...your body isn't designed to stay that way for hours.
Adjust your body position occasionally (you do this in your sleep anyway). On long flights especially...
Stretch, move your toes in circles, anything you can do in your seat. Some airlines now periodically show video programs showing how to exercise in your seat. Follow them, or do your own thing (or both). By changing position, and moving around a little, you make sure every part of your body gets the circulation it needs, e.g., to avoid deep vein thrombosis (DTV). Consider walking around the aircraft occasionally during very long flights.
Remove your shoes if convenient. On very long flights, better airlines offer slippers, but you'll also need warm socks.
With the exception of some private charters, smoking is not allowed on any commercial flight worldwide. Do not smoke at your seat, in the lavatories or tamper with the smoke detectors in the lavatories - at minimum you'll be in trouble with the airline and in some jurisdictions subject to prosecution:
In the United States federal law prohibits tampering with, disabling, or destroying smoke detectors in aircraft lavatories. And attempting to do this CAN make you get an fine of $2,200.
In almost all US Airline carriers (including American, Delta, United, Southwest, and JetBlue), Electronic Cigarettes (or E-Cigs), E-pipes, E-cigars, or any device that is stimulates smoking is BANNED on all of their flights.
Ensure that you stow any loose items before landing. Put magazines and books in the seat pocket. Don't leave any lying on an empty seat or loose under your feet. Under heavy braking on landing, even light articles can travel quite far forward. Those items can be difficult to find, and before you find them, they may be accidentally damaged by exiting passengers.
Always follow the instructions of flight attendants, as well as lighted and posted signs...there for your safety and all others.
Flight attendants are responsible for your safety, and intensely so trained.
Do not argue with them. Resolve any conflict you have with their orders on the ground.
If an "unruly passenger" becomes a serious problem, the captain of the plane may make an unscheduled landing, and the passenger will be taken off the airplane under arrest.
Flight attendant instructions are often backed by law, e.g., in the US, disobeying a flight attendant is a felony. Short of that, you still face severe fines and costs.
Mobile phones generally cannot be used in flight, unless they are placed in an aircraft safe mode prior to take-off. One notable exception are the all business class A318's that run between New York JFK and London City Airport for British Airways: mobiles are allowed on this route, but for texting and internet access only - no voice calls.
Other electronic devices are usually permitted once the aircraft is at cruise and the seatbelt sign is switched off. Rules regarding when and which devices can be used varies by country and airline:
In Australia and the United States, airlines typically give you a list of categories of devices that cannot be used (cell phones, remote controlled toys, etc...) with anything not falling into those categories being permitted once the seat-belt sign is turned off.
In Canada, however, aviation law requires a more precautionary approach. No electronic devices are permitted unless a crew member individually checks and authorizes their use.
Laptops and tablets are almost always permitted as long as you get a flight attendant's OK and any Wi-Fi capabilities are disabled. Accessories (external hard drives, mice, keyboards, etc...) are not allowed by law - Air Canada used to specifically mention that in their safety briefings.
iPods, media players, and game systems are usually fine, but again you must ask first.
Other devices are at the discretion of the Captain or flight attendants.
Ireland does not permit the use of any electronics on flights under 90 minutes.
Meals and Delays
Scheduled meals (if any) will often be timed and typed to complement the time zone of the flight's destination. As a result, first or early servings may not match your departure time. For flights that promise no food during meal hours, consider buying something at the airport (in the secure area); most lines will allow you to carry it on-board.
On-board meals for some airlines may be brought in from one of its base or hub airports rather than from a local source. This takes considerable time. Meals kept too long for any reason may have to be discarded due to safety. Scheduled meals may then be limited to packaged snacks/cookies and drinks... not the fault of the crew.
Deep Vein Thrombosis aka DVT
The following discussion should not be used as medical advice. Consult your doctor if you think you might have or be at risk for DVT during planned travel.
Passengers on long flights can be prone to DVT, which is essentially blood clots forming in the veins, especially those in the legs. It actually affects anyone who remains seated for long periods, e.g., train travelers, car passengers. You can take precautions to avoid it:
Stretch and/or walk every 2 hours or so. Even a trip to the washroom and back is better than nothing.
Do in-seat exercises, e.g., flexing legs and torso, rotating/flexing feet.
Prefer aisle seats that will let you more easily get up and move around.
Stay hydrated by drinking water, or other non-alcoholic, caffeine-free beverages.
Consult you doctor to see if certain preventative measures are needed for you. They may include wearing calf-length compression hose (ask about the right kinds), perhaps taking a blood-thinner (e.g., simple infant dose of aspirin) shortly before your flight.
Be aware of the early symptoms (e.g., pain or swelling in the legs) which you may notice during flight or later. If suspected, seek medical treatment promptly. If neglected, the condition can become serious, even fatal.