In cramped conditions, 35,000 feet above sea level, good manners are paramount.
Do not intrude on to your fellow passengers' territory: keep elbows firmly tucked in; ease your chair gently into a reclining position, which will avoid a sudden invasion of the limited legroom of the passenger behind.
On long haul journeys, be aware of the people around you. Don't sprawl and lounge in their space; keep noise from headphones and hand-held games consoles to the minimum.
If you have children, ensure that they do not kick, jolt, or otherwise interfere with, the seat in front. If you're unlucky enough to have a screaming child on your hands, try getting up and walking around - cabin space is admittedly limited, but a child will generally respond well to a change of scene.
Drink in moderation; boisterous behaviour will irritate your fellow passengers.
Friendly conversation with your neighbours can be enjoyable, but choose your topics. Many people are terrified of flying, and will not take kindly to jokes about turbulence, hijackers or the competence of the flight crew; such talk may even get you arrested.
Stay relaxed when embarking or disembarking. Help the elderly (or those of diminutive stature) to stow (or remove) luggage in the overhead lockers.
Behave courteously towards the flight crew and thank them for their service during the flight.
Do not barge your way to the exit as the plane doors open - even the sharpest elbows will not ensure that you reach the terminal any faster.
at the airport
It is polite to others to be organised: queue patiently, have the relevant documentation to hand and be prepared for security checks.
In airport lounges, respect the ambience and atmosphere. If fellow-passengers are attempting to work, then loud conversations (with friends and colleagues, or on mobiles) should be avoided. Similarly, avoid conducting confidential or sensitive business calls in earshot of fellow passengers.