goodluck getting to it. My advice would be to use your left forearm (if right handed) almost as bait. Pulling away from it will be useless and is the automatic response, so you have to get in close and pound at it with your other hand. Unlike a cat, although a dog has claws enough to scratch you, they won't be ripping your inards out, so the mouth is the only real threat. If you could get it in a choke that would be idea but you'd have to get in really close because they could probably easily slip out of it.
Basically though dogs are strong but stupid, so I imagine with some training most people could defend themselves against most dogs.