Some of the above comments are good, some are not so good. I've had police officers tell me what to do, and I saw one of my MMA friends restrain a dog with a RNC so here is my experience and knowledge:
*Wrap a shirt or jacket (Preferably a durable, leather jacket) around one arm. If he bites that arm, shove it all the way down his throat, behind his teeth. The dog won't be able to close his jaw, but he won't want to release -dog's are generally too stubborn for that. You'll have a free hand. The other hand will need medical attention.
*The best position to be is behind the dog. From there you can apply a Rear Naked Choke. You can choke a dog with it too. I've seen this successfully done in a street fight situation, but I wouldn't recommend it too much, unless you're good at getting position.
*The best tip is to avoid fighting with any animals whatsoever. They're ultimately better equipped to deal with violence than you are. Generally, animals are armed. Their claws and teeth are good weapons for them. We have our brains instead. Please use that and avoid any harm to animals.
I hope this helps.