You can also grab their tongue and pull as hard as you can. Apparently that prevents them from biting down on you, but also means that you have to stick your hand near their mouth to grab it first.
What you do once you've managed to get a good grip and pull the tongue out is beyond me though!
Very difficult. When younger i used to try it when we had the dog in the car and she liked putting her head out the window when driving along and panting