As soon as he runs out the slack in the leash, make like a tree. Don't speak to him and don't make eye contact. Simply stand your ground and ignore him for a moment. If he strains at the leash move just a few inches to give him some more slack and then stand still again. Depending on how stubborn he is or how interesting what he's trying to get to smells, he will soon get the idea that the walk is over until he begins to walk again at your side. Once he does try to pick up the pace and praise him for keeping your pace.

 

While working on your loose leash walking exercises you need to understand that it takes a good long while to get a loose leash and that stopping the sniffing is a completely different part of training...working on leave it in less distracting environments first and building up a reinforcement history while gradually building up to the sniffs outside will be necessary to get a good leave it. You can also use the sniffing ITSELF as a reward for LLW eventually, which will give you the opportunity to put "go sniff' on cue (making it less likely to happen on it's own) AND improve his leash manners.

 

Yep this is what we do, teach the dogs a 'go sniff' cue and a 'leave it/off', works like a charm

 

You could try using a harness with the lead clipped to the front ring. This will take away much of the dogs pulling power and not hurt his neck. And yeah, be a tree. No jerking needed.

 

I've found that some dogs (especially large-ish dogs with a habit of pulling) don't necessarily "get" the "be a tree" thing. In that case, I recommend either backing up or turning in the opposite direction when you see the dog start to focus elsewhere (before he runs out of leash). You may not get very far, but you will get as much "exercise" - just in a smaller area. I'll also allow the sniff to be a reward, but my feet only go THAT direction when the leash is loose. Try to work on this first in low distraction situations, with an obvious draw (treat or toy on the floor, person the dog wants to see, etc.) and ONLY a loose leash gets forward motion, otherwise you are headed elsewhere. If you HAVE to walk the dog, and don't have time to train the correct behavior first, try a front-attach harness, or harness with two points of connection for potty walks while you work on nice LLW with different equipment. The other thing I've noticed is that lots of times, when the dog is a puller, the owner is a puller too, and the simple act of making sure YOU are keeping a loose leash makes a huge difference. Also, TEACH him to give you attention and teach him a good solid leave-it in easier-to-control situations so when you need it, you will have it. He doesn't have to stare at you the entire walk - just keep the leash loose - but I've found you get much better results if you never start moving with a dog unless he first gives you eye contact.