Mug them before they mug you...
as a seasoned geese-mugger I suggest that you should strike first and let them know who's boss! I've found in my experience that grabbing the ring leader round the neck is often sufficiently scary to the rest of the muggers that they flap their wings and disperse to the far side of the pond. Of course you shouldn't try this unless you are confident in your abilities; I suggest that you start on ducks and move up to geese. Eventually you may feel confident enough to try swans!
The technique I prefer is to lull the victim into a false sense of security so that they feel as thought they have the upper hand. It's very much the same no matter what sort of animal you want to mug; if you adopt a low stance the victim will think that you are less of a threat. So, squat down to geese level and use your weak hand to simulate a goose neck / head. The muggee will give your hand plenty of attention especially if you move it around in an avian manner. Whilst this is going on you should strike out with your other hand and grab the bugger round the neck (close enough to his head so he can't peck you). Once you've caught him you should try to subdue him quickly so he can't fight back. I prefer not to harm animals, so I would suggest nothing more than covering the goose to prevent it from wildly flapping and perhaps hurting itself or you!