One of the main obstacles I find myself facing when shooting airshows is when I track the plane and start shooting I tend to lose the plane in the finder and get half shots, corner shots, etc.
This is a phenomenon know as a Jacobson, where you end up with shots with cut off noses, tails or wings....sadly I know it too well.
As for advice for reducing them....as Nick said, going a bit eeasier on the zoom is one. Or go and practice. Either on GA aircraft when you can or some people recommend cars on the highway, but I cant do that too boring.