No matter how cute Rover looks with his shiny new collar and harness he's still a dog, and that means he will behave instinctively if you do not teach him acceptable behavior. Dogs are pack animals and as a part of establishing their dominance in the pack they may exhibit aggressive behavior toward other dogs. The longer a dog gets away with behaving aggressively, the more difficult it may be to train him, although nearly every dog can learn to control aggression with consistent and firm instruction.
Learn to recognize your dog's bodily signs of agitation. He may stare directly at another dog and the fur on the back of his neck may bristle. His ears may flatten backward and he may tilt his head slightly but never take his eyes off the other dog. If intervention does not occur, your dog may begin to growl or he may attack the other dog.
Put safety first if you have an aggressive dog. To prevent an injury to another dog, muzzle your dog. After he learns to react without aggression, you may remove the muzzle while walking him or taking him to the dog park. You can still train your dog while his muzzle is on.
Socialize your dog as much as possible but keep him on a short leash. The more your dog encounters other dogs, the less likely he is to feel immediate aggression when he spots one. Keep your dog on your left side, with his head level with your hip and wind any excess leash around your wrist. If your dog lunges towards another dog, issue a firm command of "No" and pull back and downward on his leash. Avoid pulling back and upward since this puts your dog in a more aggressive "chest up" stance.
Instruct your dog to sit down beside the sidewalk if another person and her dog are approaching from the opposite direction. Stand directly in front of your dog as the other dog passes, blocking your dog's view, and speak calmly to him to keep his attention. When the other dog is a safe distance away, give your dog a treat and praise him for sitting still. When your dog masters this task, allow him to continue walking, always keeping yourself between your dog and the other dog.
Control your own reaction to your dog's aggressive behavior. Issue commands firmly but do not yell. A dog can sense anxiety in his owner and your nervousness may heighten his tension.
Buy a gentle leader for your dog. These specially designed head halters pull the dog's head downward when you pull on the leash. As soon as your dog exhibits the slightest aggression, pull gently but firmly on his leash and the gentle leader will make him lower his head, breaking his eye contact with the other dog and averting a fight. (See Resources)