Our Aussie had this problem as a pup, and what Carrie suggested worked wonders. He was so strong and boisterous from the time he was about 5 months old, and would simply get too excited to walk nicely. When he'd chew the leash or start hauling us down the sidewalk, we'd "make like a tree"--stand perfectly still, not look at him, and wait for him to come to our side and sit. He figured things out pretty fast and we've not had this problem for months!