For all the variety in ways that we will be able to train our dogs, there are a number of bad, even violent systems that folks erroneously use thinking that they will somehow be good for the dog in the longer term, or because it’s better to get an instant response using negative attention and the person has run out of patience with their dog training methods.
Patience is critical, so if you're growing irritated with your dog during a session and find that you are letting your feelings show, simply stop your dog training session and try again when you have calmed down.
Employ a positive tone when commanding your dog to sit, stop barking, or come to you. If you train them using a negative tone, they will learn to follow your commands reluctantly, feeling that they're getting it wrong. Consistency is so important with dog obedience training, so even if your dog is being bad you ought to use the same, positive tone to order them as you do in your regular dog training sessions.
More importantly, there are some methodologies which are frankly plain aggressive, eg rubbing a dog’s muzzle in pee or feces to hurt them for an accident. If your dog has been housebroken, an accident inside is either a sign that they aren't getting out enough, or that they might be sick. Either way, they shouldn't be punished.
You shouldn't forcefully yank on your dog’s leash or collar. It can feel instinctive and natural to withstand a dog’s misbehavior by tugging on their leash, but it’s important to train yourself out of this behavior. You want to teach your dog to need to do things properly, not battle each other.
Don't shout at your dog for any reason. Further, avoid forcibly leaning in to your dog’s personal space, as this is terribly threatening. You would like to promote a powerful, friendly bond for successful dog training.