G'day Cindy,
that question is one of the most commonly asked, with a similarly sized plethora of possible solutions.
Many will tell you to try this driving bit or that piece of headgear, and sometimes some of that will help, however that usually depends on how long the horse has been in the "habit", cos more often than not, the habit was formed early in the horses working life and has gotten worse rather than better with time.
I have found greater success with trying to firstly understand what situations have triggered the onset of the habit and then designing strategies around re-skilling the horse...ie; allowing him/her to learn different reactions to well known situations...this is not guaranteed to work...however if you are patient, observant, open minded and in tune with your horse your chances of success are greatly enhanced.
One example is a horse that came into my yard, who was trained previously by the people who bred him, was well cared for by those people and they had raced him quite successfully down to a tight mark in the country, however his pulling habit meant he was not competitive in Sydney ( sorry I'm in Aus) where the competitiion is quite a bit harder.
The horse used to fire up when another horse worked near him, so rather than avoid that situation we worked him every day ( to start with in his slow work) with at least one other horse right up against him, when he started to relax with one horse near, we brought another horse or two in and gradually over about six weeks of jogging the horse is jogging around very relaxed with all his new buddies... we then took it small steps at a time with his faster work, while we were building him up to race fitness, sure there were some little triggers that we had to avoid along the way, and that's all about observing the horse and understanding what is setting this off, but hey! we gave him his first start and he ran a great race in beating all but the winner and then off he went and was a great horse to us winning several times in Sydney against horses that were leaving him for dead previously, in fact MANY people asked the question "how did you stop him pulling?" and my stock answer was "worked on his mind a little bit". that horse was only one of quite a few we have turned around...you just need to listen to the horse, he will tell you what is setting him off if you take the time read him.
cheers,
Dan