Most dog training has to be done in stages. You have to teach the dog a little bit at a time until he grasps the overall concept you want him to learn. An example of this would be teaching him/her to stay. Once your dog has learned to stay in a training class or at home, it is a good idea to take it to the next training level by taking your dog to park or shopping area where there are lots of dogs or people.
You do want to obey leash laws, of course, and keep your dog safe, so keep your dog on a long (25-30 foot) line in case you lose his attention and he decides to take off.
With your dog in a “down-stay” or “sit- stay” position you would walk about five feet away from him and then back. Then slowly increase the distance and duration of the stay. If your dog does well you can keep increasing the distance an duration and even move to a location with more distractions. If your dog is having trouble you can go back and make it easier until he learns what you want. If the distractions prove to much you would move to a more quite place for a while. Be flexible when training your dog.
After each success you can go back and reward him or call him to you. Vary it so he has to keep paying attention and always reward your dog for a good stay with treats and/or praise during training.
From here you can go on to more advanced work such as a “down-stay” and walk away and hide or “down-stay” and there are people walking between you and your dog. These are more advanced so you will have to work within what your dog is capable of successfully performing
But the key is to take things in stages with what they are able to do well and work up from there.
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