I knoew this has been dead for a while, but I wanted to throw in my experience and thoughts.
One, the arm shoving technique will work most of the time. If you are dealing with an trained attack dog, ( assuming you didn't ask for it, so bad guys turn your head for a moment) it will in all likely hood attack your ankle or hold onto your clothing, they are trained in some cases not to attack the arms for this reason. Also if the right dog (such as in the OP's case a pit bull) getsd your arm in its mouth and its serious about it, there may be no shoving anything anywhere. It could break your arm and pierce your muscle and bone, holding your arm in place not to mention the pain from such a bite.
There are a couple things I have used and seen used effectively and a couple techniques I have read that make sense. One the agressive behavior and fast movement approach works pretty well to prevent most attacks, but if you are already being attacked, you may have to back it up because the dog either sees you as inferior to it or you are invading its space without having established dominanace or it has been trained to attack and ordered to attack you for whatever reason. This sums up most dog attacks.
So here's is something I have used personally and have had firends use with great success.
A typical mail order, or gun show, or pawn shop stun gun.
Size and power doesn't mattrer, you probably won't have to touch the dog with it. Just test one and get the loudest one that is of a size that you could carry. And make sure it has a well designed safety and isn't too hard or easy to activate. I have used this and this is how it works, an dog starts acting agressive toward me. If it gets close enough to me that I fear for an attack, out comes the stun gun and I just activate it in the air. The dog usually runs, not walks away. I have never had to touch a dog with it, but since they are theoretically designed to drop a thug, if you had to, it would more than likely work. In all cases I have either used or had a friend use it, the dog left when it heard the zap sound the stun gun makes. If you carried this on the job, I would recommend carrying it scout style in horizontal sheath in the middle of the back of your belt and make sure you can get to it with either hand, so a smaller slimmer model would be good for this. You could carry it in a breast pocket too and get to it easily with either hand. Tasers (the actual brand name AirTaser) probably aren't the best idea for this, though the cicvilian model, is the size of a cell phone and can be carried easily in the ways I mentioned. I believe the delux model has a flash light and has the option of being used in a close contact mode like a traditional stun gun or being fired into a target the way people are most familiar with. The firing option could be problematic for dog defense due to the smaller target and the location of the target and its capability of quickness and manuverability. A good bright flash light if used at night can break off agressive behavior and is even better if you flash the light rapidly on and off in the dogs face. May not work all the time, but I have had good results.
Pepper spray was first marketed as a dog repellent for mail carriers (so I was told when I was being trained in the use of it) and then was used by police. So its an effective measure too.
There are some tactics that I have read in a military manual for dealing with dog attack when a firearm is unavailable or impractical, but due to the terminal nature of these methods, I wll leave the OP and other interested people to look for and find those methods.
With all that being said, here is a better way, situational awareness is the best prevention for a dog attack. Dogs usually attack when threatened, cornered or in the right situation, when trained and ordered to do so.
Try to avoid threatening the dog or encroaching on its territory without acertaining if there is a dog that lives there and conferring with the owner as to how to deal with the animal..
Its pretty easy to avoid putting yourself in a situation where a dog may be ordered to attack you, and if you are in that situation, you have more problems than just the dog. The methods I mentioned above will work, but it is better to avoid having to use them by avoiding provoking a dog either on purpose or by accident, that way you don't have to get bitten, hurt an animal that is doing what it sees as the right thing to do, and you don't have to deal with the pain in the ass of angry irrational owners, bites, police and the whole rigamarole that is a dog attack.