I know a bit about this as my parents raise german shepherds, some have which have gone through attack dog and police training.
Dogs (like humans) are not born with the ability to fight. They must be trained or learn the hard way to fight effectively, this includes biting. Sure, they are all physically capable of it, but not all know how to do it properly. Most often, dog bites are like punches thrown by newbies, cute and irritating but rather ineffectual.
In my experience a dog gets disoriented quickly when you take it off it's feet - they don't like that. A previous poster wrote that they will go for the closest target, usually your arm (because most people hold their arms out to keep the attacking dog away). I've found that you open up your arms, kinda like you are opening up for a hug. The goal is to draw the dog just close enough to reach in and grab his neck from both sides for a takedown. Make sure that takedown is fast and hard, and get him on his back.
Dogs are hereditary pack animals by nature and being on top and holding them down establishes you as dominant. You may need to hold them down for a few minutes, but once they are helpless they will realize you are in charge.
This isn't hard to do, but overcoming your fear in the face of growling, barking and flashing teeth is a different story. If the dog does get his teeth on your arm, jamming it into his throat does work. Remember here that his greatest (in fact only) weapon is immobilized so use that time to get him on his back and get on top of him.