well, it's a question of both knowing a good spot and having the right kit.
For example, that American 767 of mine was taken with a long lens, which compresses perspective and fills the frame with the aircraft, even though the place I was photographing from was pretty close anyway. In that case, the technique is to point the lens at the camera, and wait for the aircraft to turn across the frame until you have the details you want in shot.
Now, the Air Canada takeoff pic, that's a question of finding an unobstructed spot a little away from the runway end. Then, depending on how far you are away from the runway centreline, and how long a lens you have on, that will vary the angle the aircraft appears in the pic. So, Stephan's pic, he's fairly close, and the angle is clearly from beneath. Compare that with my shot reporduced below, where I am around 1/3 mile away and using a longer lens (500mm in this case) - you can see the aircraft doesn't appear to be so far above.
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Photo © Chris Sheldon
The problem with taking this particular type of shot is that every aircraft will be at a slightly different height as it comes past you, and if you're set up in the right spot to get heavies (747,777,A330/340) then everything smaller will be way too high to photograph.
Jason's shot of the Cathay A340 is not taken using a particularly long lens (perhaps you can confirm this, Jason?) - and this type of shot is all about knowing a spot where you can get REALLY close. There are plenty of airports where there's a spot near a taxyway somewhere that lets you get really close to the aircraft, and sometimes you'll even need a wideangle lens to get it all in. Generally, shots taken with a "standard zoom" (by which I mean a lens in the range of around 28-80mm) will look sharper and with better colours than when you have had to use a long lens. Lenses in the shorter zoom range will generally give better contrast performance and less flare than longer zooms, and if you can find a spot where a standard 50mm will do the job, then you're looking at very good quality and extremely sharp images indeed.