The incoming overhead shot is one of the most fun. The basic idea is to cover the bird with the barrel. Swing up from behind, and when the bird disappears, shoot and follow through. For a better description, see 'The Better Shot' by Ken Davies.
One often overlooked point is that a shotgun has a little built in lead when fired at high elevation at an incomer.
Wycamp Lake Club here in northern Michigan has such a shot set up on the SC course. You are down in a genuine English style 'butt'. The window is pretty darn small, and the targets must be taken nearly directly overhead, certainly not less than 70 degrees elevation for even the quickest of shots. It's really fun, and doesn't take long to master.
