Coming from a Marine Corps Background I've had the privilege of having some advanced H-H combat training and dogs had been covered...
What is said above is true...but the most effect way to handle a dog is use striking moves as it comes in aiming for the nose (the most sensitive area on a dog, more so then the family jewels) or the throat. If you miss or don't hit with enough power, which will almost always be the case with a large dog. your going to end up grappling the thing. Your best action is give it your fist...like Shamrock11 said they go for the first thing they see. So offer your fist straight out...your going to get bit...but the idea is to shove your fist down it's throat and gag it. If you get it down far enough quickly enough, it can't clamp down. It's stick and brutal, but remember it's the one trying to bite your neck off.