We use a choke chain on Harley, it's a chain with 2 rings at teach end, you basicly put the chain through one ring so it makes a loop of chain (kinda like a lasoo sort of loop, where one bit moves easily to make the loop smaller or bigger), then put it over your dog's head and clip his/her lead to the moving bit or where you would put your hands if it was a lasoo....I'm rubbish at explaning this but you get the picture.
This sort of slip chain tightens up when the dog starts to pull and then loostens off if s/he is walking calmly. I've used it on all of my dogs and the RSPCA centre suggested a similar type of thing. With the choke chain (a bad name for it if you ask me!) it also means that the lead is not attached to the dog collar so if s/he does manage to escape from it somehow -the lead snaps or something- then s/he will still have her/his collar with ID tags on.
Hope that helps, I'm sorry I'm rubbish at expaining how it works but ask at a pet shop or wherever you get it and I'm sure they'll show you. It sounds kinda cruel but it's not I assure you, becuase they soon learn that if they don't want it tight then they dont pull!