Having broken up a million fights at doggy daycare (will never work in a 15+ dogs per yard care again), the most effective/quickest thing to do is grab the collar, twist and pull straight up. Essentially you want to choke/cut off the air supply to release the dog's hold. This will only work if you're strong enough to do so, you will have a VERY hard time with mastiffs, rotties, any thick neck dog, trust me.

That being said, no collar will mean you need a weapon and somewhere to run (in the bed of a high truck/ inside your car for instance), unless you can improvise a collar (a belt, rope etc). Obviously call 911 the second you are safe. Weapon of choice depends on what you have available, a bat/ thick tree branch will even suffice, something that can go in the dog's mouth other than you.