We have travelled with our children since they were a few weeks old- not regularly enough for them to be used to it, but maybe once a year.
The secret with mine was distraction and timing. We always timed our flights to coincide with their sleeping times whenever possible.
Our worst flight ever was a four one to Turkey taken at an unsuitable time (arriving around midnight) into a crowded airport and interminable queues for customs with our then 3 week old and 21 month old older children. Luckily they both slept through the flight- the arrival was the worst bit.
We made sure they had eaten before take-off to remove hunger as a cause of unhappiness. We had ready snacks to whip out at a moment's notice, milk all ready if no longer breastfed.
We travelled to LA from London (12 hour flight) when our two oldest were 2yr 0 months and 3 yrs 9 months. The trick that time was a fairly large number of small toys purchased expressly for the trip, and placed in a small toy suitcase. We whipped one of these out whenever trouble loomed. It worked a treat, especially if you can talk to your child and explain what's happening and why and why they need to be good.
Ours understood pretty well and went to sleep when they got bored, but again, it was an overnight flight. Also, on long flights, don't be afraid of getting up and walking to the end of the plane and back. It's recommended to prevent DVT anyway, and on large planes there is plenty of room for wandering.
Another option is drugging: Phenergan (sp?)which we mooted but never had to resort to, or even Calpol if makes them sleepy, which we did resort to on occasion.