first off... accept that your going to get bitten... and its going to hurt alot... :icon_twis
now... dogs are not like other wild animal in the fact that they are not going to go for the throat... wolves and lions,tigers and such will always attack the throat first..
dogs will instinctivly go for whatever is closest to them... that's why you see police dog training vids of the guy wearing the protective suite.... the arms are the most heavily padded..
they do that because its instinct for use to raise our arms to protect the face... so the dog goes for the arm as its the closest body part to them...
example-
now in a self-defense situation, you can use that to your advantage... if you have a jacket or sweater... quickly take it off and wrap it around your forearm of your off hand...
when the dog attacks... let him first bite you on said protected (hopefuly!) forearm... then immediately try to grab his head/body and pull it into you close...DO NOT, I REPEAT, DO NOT try to pull your arm or whatever he has in his mouth away from the dog... this is where most of the damage happens.
the closer you are to the dog and the more control you can have over his head, body and movements the less you will get injured... it will not be good if he gets a mouthfull of you and starts pulling away and ripping hunks of flesh off of you... you do not want it to start pulling away on you!
I should say first that if you can get some kind of weapon, that will be much better... even a pen or some keys in your fist, a strong stick, stone, ect will hopefully keep him away from you long enough for you to hurt the dog enough or get somewhere safe..
ok.. so hes got a mouthful of forearm... you've grabbed the head... now push your arm/hand deeper into its jaws while holding the head... this will prevent him from biting down harder.. then try and jam your thumbs into the eye sockets, ears, grabbing the throat and squeezing really hard, hitting the throat.. Ive even heard of guys grabbing the upper and lower jaw and pulling them apart... tearing and breaking the jaw bones...
if your hand gets into his mouth, then thats actually better because you will now force your hand down its gullet and start ripping out anything inside of the dog... and choking it.
for a good example of what a typical dog attack looks like and what NOT to do... check this vid out..
YouTube - Pitbull Attacks Animal Control Officer! FULL VERSION!