Well, since I have my D60 I try to take as many shots as possible. As simple as that. This doesn't mean that I press the shutter release button and just let it take 8 pictures. No, sometimes I take 3 or 4 when the a/c is coming towards me, and when it's side on I take another 2 or 3, and when the aircraft is about to touch down, maybe another one or two.
When the aircraft is in front of me standing still, I usually try to play with the exposure, to get at least one good result. Bracketing might help.
And, untill a couple of weeks, sometimes I didn't take a shot of an aircraft, e.g. when I've seen it 100 times before. However, now I take at least 1 shot of every aircraft that flies over. You never know if something might happen (let's hope not!). If it does, at least there'll be at least one final picture of the aircraft.
Hope this helps,
Ivan Coninx - Brussels Aviation Photography