At the airport
Most international airport terminals are standardised, with similar procedures for check-in, security, immigration and customs. English is the standard language, if not another European language (French or Spanish).
If you want to reduce stress get to the airport at least an hour before the recommended time. (Check with your airline. In the US, the recommended time is usually 1 hour before takeoff for domestic flights, 2 hours for international. In some countries, it may be up to 3.) You will not have to worry while standing in long queues for check-in, security, emigration, and more security. It also gives you a buffer for delays on the way to the airport.
If for some reason you are delayed and you're worried about missing your flight or the flight status indicates that you are in danger of missing your flight, find a member of your airline's staff or talk to staff at the security gate. If you are really in danger of missing your flight, they can arrange for speedy check-ins and for you to be moved up in queues. But they won't notice if you don't tell them. Calling for late-passenger instructions while you are on your way to the airport can also help. The plane will not wait for you; but it might wait if you're one of 50 connecting passengers on a delayed flight.
Check-in for domestic flights can usually be done on the airline website up to 24 hours in advance of departure. If you have no baggage you can just proceed directly to your gate and flight with your printed boarding pass. However, some carriers insist that they inspect and verify your travel documents before allowing you to go through security, do this but there is usually a special lane provided for you. If you have baggage, drop it at the bag drop lane. Removing old tags from your bag before proceeding to the bag drop will speed up this process and avoid redirection.
If you can't check-in online, the check-in kiosks at the airport are much the same, and issue a boarding pass for you. You then need to go to the bag drop if you have more than carry on luggage.
If you have to check-in manually, be prepared for longer queues. Have your documentation ready before you get to the counter. If other methods of check-in are made available, avoid using the traditional check-in counters unless you have special requests. Some carriers already charge a fee for using traditional check-in counters.
See also choosing aircraft seat.
Express/expedited security lanes
To avoid the delays associated with normal security checks, some airports offer express security lanes for frequent travellers who have pre-registered, or sometimes for passengers who have paid an additional fee.
In cases of heightened security, the expedited security check lanes may be suspended or closed.
Even if you don't hold a first/business class ticket or are a member of the premium tiers of your frequent flyer program, there are ways for you to obtain lounge access:
When a flight is cancelled, the reason given is usually some kind of technical or weather-related problem. Sometimes the real reason is that so few passengers have checked in that it is cheaper for the airline to cancel the flight and rebook the passengers on a later flight, or even on another airline. If a flight is cancelled, the airline is obligated to get you on the next available flight to your destination, but interpretations of "next available" vary and, for some low-cost carriers like Ryanair, this may mean a long wait indeed. Unlike with overbooking, passengers are not legally entitled to any compensation except the unplanned expenses of food and hotels. Within the European Union, the same compensations like denied boardings apply, unless you have been informed more than 14 days in advance or the airline claims they're not at fault like weather conditions (which they typically do).
Beware that weather can cause the very strange phenomenon of being denied boarding because of weather for a flight that does depart on-schedule. This is usually caused by weight limits and takes two forms:
1) Predicted weather may make the flight longer, and so increase required reserve fuel. Most planes can't take a full load of passengers and full fuel; if they must fill the tanks more than expected, they might have to leave some people behind.
2) As it gets warmer the takeoff roll increases (the air is less dense and so decreases wing lift as it slightly decreases engine thrust) but the runway doesn't get any longer. If the air temperature gets hot enough, they may have to reduce weight for the plane to get safely in the air.
Occasionally flights are delayed... for many reasons, e.g.:
Airlines never unnecessarily cancel or delay flights... it costs too much, in money, perturbs many other flight schedules, and generates poor public relations. When they do delay or cancel, they usually go to great lengths to arrange seats on another flight, sometimes even on another airline. If a cancellation has been caused by them, they are required by law to pay you certain compensations and/or arrange lodging and/or meals until you can be flown to your destination.
Often a boarding order is specified by the gate attendants, usually boarding business class, passengers with special needs, and passengers at the back of the plane first. When no boarding order is given it may help if those seated at the back were to board first, but this doesn't usually happen, and aisle blockages are common. To estimate where your seat is, check your airline's website for seatmaps or ask staff at the gate. Regardless of the boarding order given, you are always free to remain in the boarding lounge until the final call for the plane. If you choose to spend the least time possible in a cramped aircraft cabin, just wait in the boarding lounge until you see the last person at the gate, and join the end of the queue. Just remember, the boarding gates close 10-15 minutes before departure and no announcements will be made outside the gate area.
If you arrive at an international terminal for transfer to another flight, you usually do not need to pass immigration. However, passing security is usually required.
Passing immigration booth can take long time. Have your documents ready, and listen carefully to official instructions.