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What is THE most underutilized cue? Focus. While it is one of the first new behaviors taught in basic obedience it seems to fade away once sits, downs and stays become our "go to"s. It shouldn't, tho. A solid focus means that no matter the distraction the dog's attention will be on you. It helps develop self control and calmness.
Does your dog have a trigger that keeps him from listening to you? He sees another dog/person/rabbit while on leash and his manners go out the window? With a solid focus when you see the trigger you can ask for his attention until the trigger is gone.
Practice focus everywhere. Start with no distractions and then add as many environments as possible until the behavior is generalized (dog trainer speak for a behavior that is consistent everywhere).
Start with high value treats initially and big praise. Make sure you always praise or give a life reward when it is successful. Remember, they need a reason to look at you rather than the distraction.
At home ask for it randomly while you are watching tv, fixing dinner, reading... Say the dogs name and then ask for focus, using the treat to lure the dogs gaze to yours. Take the behavior from room to room, adding distractions. Have your spouse or child clap, move around, open and close doors. Go outside, in your yard and randomly on walks. As with any learned behavior continue to ask for it randomly, even after it becomes automatic, otherwise may fade and not be there when you need it.
What does force free mean? It isn't just "no shock or prong collars". It isn't just treats. It is learning to communicate with the dog so that you can teach without intimidation. We want the dog to want to do it rather than be afraid of what will happen if they don't.