Yep, I mean walk backwards! Some people use the turn and walk the other way thing but I have found that this just excites an excitable dog further. Walk backwards five paces and stop. Of course the dog will be confused and probably facing the wrong way - this won't matter. Simply wait until the dog is calm, he is used to walking the way you are facing and will sort himself out. If he doesn't and is standing still, facing in either direction, carry on walking normally until he begins to pull again. If you really get in a mess with this try walking backwards and then asking the dog to sit before moving on again.
If the dog presents problems of size and strenghth get him well used to the procedure in a small enclosed space first so he is ready to walk back or use a halti, but do not correct using this. If you use a halti remember that you must react as soon as pressure is put on to it and do not be fooled that the dog is learning anything from the halti.
With young dogs or dogs that have never been taught the meaning of "heel" it is fine to use food to show them what you want them to do. For dogs who will walk perfectly well at training class but not on the street they are showing dominance and food should only be used as a reward when they get it right and not as a bribe.