I think Jo's information was right on...not to mention posting a great photo. To take it a step further, here's some additional information to consider:
Set your camera to servo or track focus- this way your camera will make minor adjustments in focus as you lock in on your subject plane. If you're using the shutter button to focus, hold it down half way to lock on the subject and then keep the plane centered in your viewfinder (and focus point) as it passes by you.
Practice your panning movement before you actually start shooting- I do a few practice "swings" to make sure I'm keeping the camera as steady as I can before the actual pass.
If there is a "sweet spot" in panning, it is when the plane is perpendicular to you. If you get the shot right, you should have nose to tail in your field of focus as well as the maximum blur effect to the background. Another advantage to this angle is that you can use a slightly higher shutter speed and the prop won't look so "frozen" because you're only seeing the side of the propeller.
Try starting out with, say, 1/250 of a sec for shutter speed and see what you get. If you seem to be getting the plane in focus, then slow the shutter speed down to 1/125 and try that. If you can get a shot like Jo's then I think you're there!
Panning is all a game of percentages to me. You will get out-of-focus photos. But remember the ones that count are the in-focus ones. One good picture makes up for all the ones you delete.