There are two or three harnesses that are designed for any size dog on the market, specifically designed to stop the dog pulling. One is made by Halti. They have two rings for lead attatchment one on the chest, which if the dog pulls will turn it towards the handler, and the other on the top of the harness. Most people that have used these with their larger dogs, say that they are very good. Not all dogs will take to a headcollar, and can get really stressed out, even when introduced to it gradually, so this seems a much kinder option.
Of course teaching your dog to walk on a loose lead using reward based methods are the ultimate goal, but as a training aid I think that the anti-pull harness is a great idea, allowing the handler to relax and take the stress out of taking the dog for a walk.