Basically the problem is that the dog has never been told that it is not ok to pull. They have no reason to walk to heel, but if you 'explain' to them what you want, they will soon realise and say, 'why didn't you say that in the first place!'
Whenever people complain their dog pulls them, you never see them trying to do anything about it, they just let the dog drag them along, so the dog carries on dragging them, and they allow it to. Dogs are not born knowing how to walk on a lead. They want to go for a walk, to the park or wherever and will try to get there as soon as they can.
You must show them what you want, and this should ideally start as soon as you put a lead on them.
This is where is feel show ring training has the edge over pet dog training, as most people only take their dogs to classes when they are having a problem. With show training, the pups are not allowed to pull in the first place, they are encouraged to walk or trot alongside their handlers and are given praise and titbits when they do it right.