This was one of the things they focused on in my obedience class. Since you have a door to the backyard that's fenced in it's pretty easy to practice...
Tell them to sit and stay in front of the closed door, open the door just a little and close it again. If they kept their butts on the floor, you give them a treat. If they move, tell them 'naha' or 'no' or whatever command you use for that. And if they get it, open the door a little further each time. Keep practicing until you can open the door all the way and go through without them moving. They can only come through the door when you say 'Let's go' or any other command you use for that.
It takes a lot of time and practice, but these are smart little buggers and eventually they'll get it. And it's worth the time, because it could save their lives!
__________________  Trixie |