There are many "tools" marketed for dogs which pull on the lead, from things which cause fear, pain and damage such as shock, pinch, and choke collars, to the newest kinds of head collars and gentle harnesses. Studies conducted over the world show that the majority of dogs with cervical spinal injuries have a long history of pulling on the lead, with more than average having been walked on some sort of choker collar. Choke chains have been proven to cause problems with nerves in the legs and eyes, damage to the throat and fainting amongst other things.
Choosing the correct tool to help you and your dog can be daunting, there are many companies out there offering quick fixes, but not everything works for every dog. Here is my summing up of various training aids available.
Headcollars which lead from underneath the dogs chin. (such as Gentle Leader, Halti, Dogmatic, Dogalter and equivilents) work by steering the head. Good for dogs who lunge at other dogs as they offer a slight muzzling effect. Need careful introductions (see guide to introducing headcollars) Some dogs never take to this sort of headcollar. By the action of the headcollar a badly fitting one can ride up into the dogs eyes (especially if not used correctly) This is obviously not a good thing! The best fit i have found appears to be the Dogmatic as there are many sizes. The Gentle Leader also offers a lot of adjustment. Some dogs work out how to still pull in these by bracing the neck muscles and pushing on the headcollar. If your dog does this i suggest you look at;
Headcollar which lead from behind the head. (K9 Bridle, Canny Collar, GenCon) A newer style of headcollar the action on these is easier for both the dog and owner. Still requiring a degree of introduction these do tend to be more readily accepted. When the dog pulls it pushes down on the nose in the same way as a bitless bridle for horses. The main differences are the Canny Collar the leading part forms the nose loop. Some dogs work out easily how to get the nose loop off on this one, but the dog can't get free due to the design of the collar. The collar part of the Canny collar needs to fit quite tight or else it won't work as designed. The K9 Bridle is more a one peice item with the pressure more coming from the back of the head. This means the dog can't get the nose loop off quite as easily. This one comes with a link joined in to connect it to the dogs regular collar so again the dog can't get loose. The GenCon is a figure of 8 headcollar and therefore tightens over the nose and the back of the head. It is very effective but some dogs (like mine) will still pull til they gag!
Harnesses, there are a few types of harnesses which can also help with pulling. There are things like the Sporn Non Pull which tightens under the front legs (armpits ish) and removes some of the traction from the front legs. I have found they either work or don't there is no in between some dogs just carry on pulling in them! There are harnesses that lead from the front (Easy Walk pro) which turn the dog to face you if they try to pull, and harnesses which are similar but used with a double ended lead as a steering effect. These work well with a training program but i don't find them to be a quick fix, however they are a longer term solution as with the right training program will teach the dog good lead walking.