Have the same problem with Kimmi, a 17week old blue roan girl.
Went to our lcoal pet store for advice, and they suggested a 'comfy leader' which goes around the back of the neck, and around their muzzle and when they pull, it presses on pressure points behind the ears, and releases a relaxant hormone.
We have been using this for approx 3-5 days now, and Kimmi is much better on the lead.
As for off the lead - well we have yet to try this one!!
Hope this helps.
Sarah and Kimmi