Hi Alan, Robin,
When planning to pan, place your feet where you intend to
end the pan NOT at the beginning of it. Turn your upper body to the beginning of the pan like a golfer before he swings it might seem a bit awkward but when you unwind at the point you shoot, your body will be more stable.
I would actually suggest that advice be modified slightly in as much as the pan should be continued somewhat after the shutter is tripped (or held, then released, if shooting in burst mode).
Stand to face the point where you intend to shoot (perhaps opposite an area with cleanest background), then 'wind up' by twisting body to start pan as vehicle (aeroplane, whatever) approaches, trip shutter at appropriate point, but continue pan (body twist) after 'hitting the ball'.
This technique will certainly improve upon panning results achieved when;
a) stood facing the approaching vehicle and being uncomfortably twisted by the time you get to the best angle, or
b) shuffling from foot to foot to rotate, or
c) unconsciously pre-empting the end of pan and slowing down just as you are shooting
Yup, I've done all those things in my time!
Alan - welcome to the CiC forums from me.
Cheers, Dave