Approximately half of my private dog training caseload involves aggression directed towards other dogs or people. While more difficult than basic or advanced training, my success rate for helping a dog overcome aggression is very high. I practice very measured, systematic strategies to lower dogs overall anxiety and help them learn to be comfortable in situations that currently make them aggressive.
I only recommend, and use, positive reinforcement techniques.
Mistake - Alpha Rolls
Some trainers recommend flipping a dog on his back when he shows aggression. This is called an "alpha roll". Sometimes I have heard that it is also recommended for someone to put their face close to the dog's face while they are in this position. This is named after the supposed, and incorrect, notion that the best way to modify a dog's behavior is to "show it whose boss" or "be the alpha". Trainers lacking in understanding of animal behavior think that wolves flip subordinate wolves on their backs in this position to exert dominance in a pack situation. In reality, submissive pack members will roll over on their own to show that they mean no harm.
Why It is a Mistake
You are punishing the behavior and not addressing the underlying anxiety. A dog often shows aggression because he is uncomfortable with something and wants it to move away.
For instance, if a dog growls at a child and then the dog is alpha rolled, this puts the dog in a very vulnerable position at the same distance away from the child that originally caused the anxiety and aggression. 

The dog will not become more comfortable in that situation.
He is still uncomfortable, but now he is also more vulnerable. This can increase the negative association with the child because sometimes it results in the added trauma of being rolled. It can also have the added problem of punishing signals. Barking or growling is a signal to the child to move away.
If the dog is punished for giving signals, he might stop giving signals but still remain anxious. Besides being anxious that a child is going to come too close, the dog is also anxious that he will be punished for giving signals. The dog might remain quiet in that situation, but this is a false reading of comfort around children. As the anxiety builds over repeated exposures, a dog is repeatedly put into emotional turmoil and can reach a point where he can't handle the proximity to the child and attacks, seemingly unprovoked.
A few months ago a client called me up because her dog bit her in the face after she alpha rolled him when he growled at her. She was told to roll her dog and put her face right up to the dog's face and say, "NO!". This advice was given to her by a trainer that advocates choke chains and other physical punishments.
Her dog bit her in the face, requring 5 stiches. After assessing her dog for handling, approach and resource guarding issues, we worked together over a few weeks and I showed her positive reinforcement strategies to address these issues. She has had no problems since and she also doesn't have to jerk her dog around with a choke chain. The dog isn't afraid to growl at her, he is more comfortable with the specific interactions that my client has with him.
Big difference.
