Following tips come primarily from an American karate teacher with canine experiences, and of military experts. It is not intended as a pleasant story. Animal lovers will be disgusted. But reports of people killed and maimed by dogs confirm that good information can be useful. You 'd better read it and not need it, then do not read and ever need it.

aanvalhondA dog fights often out of fear, because he feels threatened.
Not look him straight in the eye, but lost a threatening dog certainly not out of sight.
Position yourself in an angle, face to face resembles an attack posture.
Do not laugh, because you show your teeth. That seems aggressive and challenging.
Your body speaks (words say nothing). Show you're not weak or scared, do not turn your back to him, and do not walk away (he is still faster). Go running activates his hunting instinct.
If you call "Sit!", do it with a low command voice, without wheezing or hesitation.
Ball up your fists to protect your fingers. Hold your arms at your body, do not swing them.
Keep a stick, garment... between yourself and the dog. That creates some distance and is normaly his first hurdle and purpose.
Step quiet back and search for refuge behind, or in an obstacle (car...)
Up here you could try to avoid a fight. Stay alert and ready to pounce. If the attack starts is going to run no longer an option. Activate not your fears but your killer instinct. If the threater feels that you are not prey, but a threat, he possible chose wisely to avoid a fight. Do you have a weapon, use it.

If you have nothing in your hand you use (for right handed the left) arm (best with a garment around) as a defense and spacer. Then your neck is protected, you know the attack target, and you still have an arm and two legs free. Turn the outside to the front, bony edges above and below, softest side with veins to you.
Make sure you try to keep your balance, avoid falling, brace yourself.
Try not to withdraw your arm that gives larger wounds. Push as deep as possible into his throat to reduce his breathing.
Squeezing the nostrils or biting the nose a tenacious fighting dog has ever worked. The dog let loose to breathe. It only is a time saver. A pit-bull breathes and is immediately back on.
Do you have a stick, try to ram this in his throat.
Do you have a knife, cut his throat.
Attack his vulnerable eyes with your free fingers.
Shovel or squeeze hard in the balls.

Or put your free arm behind his head, ram your knee upside into his ribs and press back up his head till his neck breaks.
Or try to grab a hind leg and turn the dog on his back. Let the leg not loose, but drag the dog to a safe place.
Or panache him holding the back legs around and hit him with the head (or backbone) against a wall, tree, or stone. And again.

If you are on the ground: roll into a ball: your thighs protect your belly, your knees the throat. Your hands on your neck, elbows against your sides. If you freeze completely the fighting stops (maybe). (This is the safest attitude to protect (the weakest parts of) your body against bites, hitting, kicking.)

Another read advice is: with both hands around the neck flattening the arteries. Seems difficult and unrealistic. The dog won’t sit there waiting. You need your hands free. And where are they exactly?

If you are attacked by more than one dog you should focus on the leader (s) and eliminate him.

There is a huge difference between theory and practice. In newspapers I did not find more than 'man bites dog'.

"I’ve been bitten in my leg by a dog.'' “Did you put anything onit?" "No, he right bit like that."