For a quick fix, hold the lead about half way down in one hand, try looping the rest of the lead around the dogs chest, fairly low down. You need to keep the bit from the collar to your hand loose, so you don't choke the dog, but the bit round the chest taught, and hold the end of the lead in one hand where it joins the bit from the collar, hope this is making sense. The idea is that the dog pulls into the loop round its chest and meets the resistance from the lead.
Its the way we used to stop the greyhounds pulling at the track without making them choke themselves on the lead.