Everyone has their own views about curing aggression. A surprising number of trainers recommend abusive methods such as choke chains and shock collars. Those same trainers think that you have to treat aggression with aggression. "Show the dog who is boss" or be the "pack leader" or "dominant". I urge you to look beyond the fear tactics they use to scare you into leaving your common sense behind and hire them to beat up your dog. If it feels wrong and abusive, it probably is.
Don't let anyone tell you that you need to beat up or "break" your dog to cure him of his aggression.
Follows are some mistakes that I see when working with aggression. Want to join in on the conversation? Leave comments. I won't be shy responding.
Mistake - Using choke chains, prong collars, shock collars, or other positive punishment.
This is often incorrectly referred to as "negative reinforcement". Positive punishment is the act of adding something aversive in order to stop a behavior.
With aggression, some trainers recommend using a choke chain or shock collar and adding pain when a dog is aggressive.
Why it is a mistake.
Similar to alpha rolls, the dog can seem to be "cured" because they stop showing aggression, but the underlying anxiety can still be present. It is also unnecessary and abusive to add physical pain to any animal that is under stress. It is unfair, irresponsible and unnecessary to add pain to a dog when he is uncomfortable. Move him away, manage him better and use systematic desensitization to build confidence and get him more comfortable with the situation.
I equate punishing a dog for showing signs of aggression to punishing a person that is afraid of spiders. Just imagine if you were afraid of spiders, a spider fell on your head when you walked through it's web and you started screaming. Then, someone shocked you or choked you. You are responding to a stimulus (the spider) in way that you can't help and punished for it. If this happened, would you become more comfortable around spiders? I doubt it.
That is similar to a dog that is afraid of another dog or a person. They growl or bark because they are afraid. If they are choked or shocked when this happens, they will continue to be afraid of the approaching stimulus, and also afraid to show signals. If any animal is afraid, positive punishment can stop the animal from showing outward signs of discomfort, but the anxiety has not been addressed, except to mask it.
Then, that dog reaches a point that the dog can't help but to attack. It seems like they are attacking out of nowhere, but their signals were suppressed, so it seems like that. Not good.
Note: I am 100% against using choke chains, prong collars or shock collars in all dog training situations. Period. Comments? Please comment below.
Mistake - Moving too quickly or inappropriate expectations.
I was at a second appointment recently with a client that has a newly adopted dog that exhibits aggression towards people and dogs. She had bitten two people in the first week in her new home with little damage.
After working slowly and systematically during the first session, we made a lot of progress while I assessed Tulip's anxiety and learned her signals. By the end of the session, I could walk slowly towards her up to about 10 feet away while she remained calm the entire time. 

The second appointment we continued working and I was able to give Tulip a few treats from my outstretched hand, while Tulip was on leash the entire time.
I advised my client to move her away at some point to a distance of 5 feet. He said he was surprised that I was still cautious at this stage. She seemed fine and was taking treats gently out of my hand. 

The reason that I did not push too quickly and try and pet Tulip is that I did not want to put her in a situation where she felt nervous and that she had to defend herself by growling, barking or biting.
Aggression has obvious signals such as growling and biting, but before that happens, a dog starts to get anxious. In other words, if a dog would bite me if I touched her collar, she is not completely calm all the way until I touch her collar and then she gets instantly anxious and bites me. There is a point that she sees me approaching when she starts to get anxious.
If I push too quickly, I am proving to the dog that she needs to keep an eye on me and that I am a threat. 

I want dogs to not care about the trigger (person, dog, car, bicycle, etc.) that is currently causing stress. If a dog sees me in her house and I always push her until she shows obvious signals, then she will at some point start to get anxious as soon as she sees me.
"There's that guy, I need to be cautious, I know at some point he is going to come over here . . . oh, he stood up, is he coming over? He looked at me, this is it . . ." If that is our relationship then she will never learn to be completely calm when I am in the room or it might take a really long time.
Desensitization is working below the dog's stress level so she is not repeatedly thrust into defense or attack mode. 

Go slowly. Pay attention to your dog's signals, and it will pay off.
Oh yeah, did I mention you don't need shock collars or choke chains? You don't.
