The easy answer is never allow them to pull in the first place, We train our show dogs from the moment we get them, or even earlier if we breed them. We trot them up and down on a loose lead and praise and/or reward them if they respond. They are checked very lightly if they pull forward or back, followed by more praise, and they have no reason to pull if we make it more interesting to pay attention to us. Most show dogs go in the ring at six months old, and have been attending ringcraft classes for a few weeks beforehand, so we don't have dogs that pull.
If you get a rescue or rehomed dog then you need to start from scratch, but the biggest problem is people allow their dogs to pull them without doing anything about it. | saw a tiny Cavalier puppy on Saturday with a very large gentleman, it was pulling and choking itself and he was not doing or saying anything, just letting it tow him along.
They are not born knowing what to do, so you have to teach them. One of our customers has a ten week old Border Terrier, and they brought him to us carrying him, because they were scared to put him down. We got them to put him on the floor and put a lead on him, just following him where he wanted to go and then encouraging him back with a treat so he was quite happy to follow and didn't even realise he was on a lead.