My girl Athena is half horse or perhaps ox, LOL. Not sure but she is a big girl and would pull like crazy. She loved to pull and the harder I would try to hold her back the harder she would pull and the more she enjoyed the challenge. I trained many dogs and have always been able to break them of pulling using conventional methods and a firm voice.
Not with her though, smart as a whip but loves the pull.
I spent $17 dollars on a harness that stopped all of the pulling immediately. No special training, no getting use to it, no slip ups. The harness is like the Gentle Leader that CamiD describes above. The D hook is at the chest and nothing around the neck. That was the trick. When she didn't feel the pull on her neck her reflexes didn't tell her to pull back. She now walks right by my side always with slack in the leash.
You can check it out at your local Petco but cheaper at Amazon. Click