Keep your arms at your sides if threatened by a dog and don't make any sudden movements. Back away slowly without turning your back on the dog. Turning your back and running is an invitation for an attack.
Use any object at hand. A stick is a great weapon, as is a purse, hat or bag. Let the dog bite the object instead of you to gain more time to decide what to do next.
Use your arms to cover your face if the dog attacks. If he gets your arm, that is better than biting your face or head.
Direct pepper spray into the eyes of a dog. This will not work on all breeds, but it may buy you a few minutes.
Use a stick to hit the dog in the nose or shove it down his throat. This may disorient him for a few minutes.
Do not pull your arm out of a dog's mouth. If the dog has your arm in its jaws try and push back into the dog's mouth. This will trigger a release. If you try and pull your arm away, the dog will tear apart your arm.
Roll into the fetal position if the dog knocks you to the ground. Cover your face and head with your arms as much as possible. Keep as still and silent as you can and the dog may back off.