To add to what everyone else said, I tell my class the thing in common with all people with leash pullers: they don't talk to their dogs. conversation is VITAL. Sure you'll sound like a rambling idiot, but trust me, it'll make a huge difference. My students see an immediate difference when I send them out on their own, then I come along to I tell them to TALK to their dogs. It is IMMEDIATE.

This is what the conversation typically sounds like:
Lets go! (starting out on the same phrase every time, some people might say "come on)
Whoah, slow down.
Stay with me.
good boy.
Ut-uh... get back here.
Watch me!
Ah, thats a nice pace!
Ok, we're going to stop here on the corner.
Lets cross!
Waaaiiiittt.
Leave it!
Ooops.
This side
thats the doggy I like!
Over here

You get the idea. Its just a constant running dialog. you let your dog exactly where they stand. Are they being good? tell them! are they running ahead? Verbally catch them before your arm is being yanked out of its socket. Remember, its better to look like a rambling idiot that it is to be the joke of the neighborhood whose dog walks them.... we've all heard the comments "who's walking who?"

One thing to remember: We're trying to teach them its improper to pull us. So don't pull them back to you. Get them back any way you can. Treats, making high pitched kissy noises. Squeaker toy. anything but don't pull them back to you.

Lastly, our state of mind is VERY important. If we think our dog is going to pull us.... guess what? they will! If we step out and tell ourselves we WILL get them to cooperate, its surprising just how much better they stay with us. If you tell yourself they won't pull, that we are in control, its scary just how effective one little mental change makes on a dog. they must be psychic because they know the difference between when we think they'll pull and when we know they won't.