When I'd have a new litter, I'd tie a length of small diameter rope, with a no-slip knot, around the neck. They'd pull, tug, and stand on one another's rope, and train themselves to give to pressure on their neck; same principle as teaching a horse to ground tie by letting him step on the reins.
If I was teaching a dog to "heel", I'd snap the lead to the collar, then take a loose loop around the loins so that it tightened whenever the dog pulled ahead. Once or twice, & a half-bright dog soon figured out that if it didn't pull, it wouldn't be uncomfortable.
When I was working as a kennel tech, the only leashes we'd use were slip leads. Teaches the dog not to pull on the lead, but some people would think it cruel if they saw it in action.