Halti. Love it! My pooch is a little over 100 pounds and would pull me all the way to Canada if I let him. We struggled with harnesses and choke collars for a few months until we found this. It doesn't cause any pain, it goes around their snout and when they start to pull it simply shuts their mouth and turns their head back.
They come in all sizes and you can get one at Alaska Mill and Feed, PetZoo Animal Food Warehouse and others.