Over the years I've had a lot of questions from people with big, strong dogs who use some sort of "training collar" (prong, electronic or check chain) to prevent being over-powered. Often they have a very good reason for using this sort of collar, but want to know if they can wean their dog off it.
The answer is definitely "yes". A friend of mine weighs in at about 80lb and has two massive German Shepherds. They are both trained to walk on a loose leash, but she uses head halters on both dogs in case of emergency.
A head halter can be a very effective tool when used correctly. They are often called "power steering for dogs". They are not used for training at all, they are purely a management tool providing you with a safe means of handling a powerful dog without accidentally applying corrections.
They do need to be fitted correctly. The two I have used are the Gentle Leader and Black Dog Head Halter. Both of these collars are to be fitted very tightly high up around the base of the skull, with the nose loop fairly loose (although not so loose that it might slip over the nose). They are almost always fitted too loosely, which leads to chafing around the nose and a loss of effective control.
A viable alternative is one of the many front-attaching harnesses. I have not used one of these personally, but Sue Ailsby reckons they are great and that is enough for me!
Head halters and front-attaching harnesses are not training tools, they are management tools. For training loose leash walking you can't go past the method employed by the very same Sue Ailsby found in her Training Levels [1] program. I interviewed Sue and discovered what you really need to know to teach reliable Loose Leash Walking, the interview can be found here. [2]
A lot of loose leash problems are really what some people call "doggy zen" problems. Dogs need to learn self-control and one of the lessons we can use is called "doggy zen" - "The get XXXX, you must first give up XXXX, do YYYY, then you will get XXXX".
So if we have a dog who really, really wants to go sniff something exciting then we have to find a way to teach the dog "the sooner you stop trying to sniff that by pulling me there, the sooner we'll get there" Ailsby's "Penalty Yards" is one very effective way to teach that.