I was wondering about this just the other day. It seems that to escape once the situation has got out of control; and the dog has got its jaws clamped around you, your best bet is not to try and pull away - which would only tear flesh - but to try and push your hand/arm as far down the dog's throat as you can and choke it.
Also very important: don't let the dog ground you, or it'll have access to your neck and face.