this looks really good:

http://www.wikihow.com/Handle-a-Dog-Attack

A few key points:

Make yourself rigid and motionless. When a dog approaches, stand completely still with your hands at your sides, like a tree, and avert your eyes.[1] In many cases the dog will lose interest and walk away if you ignore him.

Do not wave your arms around or kick with your legs; the dog may perceive these actions as threatening.

Don't make eye contact, since that could also cause the dog to lunge.

Stand sideways to the dog and keep him in your peripheral vision instead of facing him and making eye contact. This will signal to the dog that you are not a threat.

Don't open your hands and arms up to a bite by extending them. Keep your fingers curled into fists to avoid getting them bitten. The dog may come quite close, even sniffing you, without actually biting.

Face the dog and command, "back away." If the dog continues to behave aggressively, and ignoring or pacifying him is no longer working, face him and sternly command him to leave.

Use a strong, deep, and confident commanding voice.

Continue to avoid making eye contact.

The dog may become discouraged or intimidated and leave.

Fight back against an attacking dog. If the dog starts biting you, you've got to defend yourself. Hit or kick the dog in the throat, nose, and the back of the head. This will stun the dog and give you time to get away.[3]

It's okay to raise your voice at this point. Yell for help as you're fighting back. Hopefully others will hear and come to your aid. However, avoid screaming as this may lead the dog to strengthen his attack.

If you have a stick or another weapon, you can (and should) use it to hit the dog. Don't hit him over the top of the head, though; most dogs have very thick skulls, so this will only serve to make the dog angrier.

Fight as though your life depends on it, because it does. Dog attacks can be fatal. While you certainly don't want to hurt a dog unnecessarily, you should use force as necessary if you are being seriously attacked.

Use your weight to your advantage. Bring your entire body weight to bear on the animal, specifically pushing down with the hard points of your knees or elbows. Dogs are vicious biters but cannot wrestle, so try to get an advantageous position and break their bones fairly quickly. Get on top of the animal and concentrate force on areas such as the throat or ribs while minding to keep your face out of clawing/biting range.

If you are looking for a more humane solution and can manage it, straddle the back of the dog with your partial body weight and apply forward pressure to the back of the neck to immobilize the dog until help comes.

Protect your face, chest, and throat. If you fall to the ground during the attack, not only is it more difficult to fight off the angry dog, but vital areas on your torso, head, and neck become more vulnerable to attack. These are the most important spots on your body to protect because bites in these places will inflict the most damage and will have the greatest chance of killing you.

Protect your vitals[4] by rolling onto your stomach, tucking your knees in, and bringing your hands (balled in fists) up to your ears.

Resist the urge to scream or roll away, as these actions may further encourage the dog.[5]