There are many different types of dog aggression and they cannot all be treated the same way. Understanding "why" your dog is aggressive is essential to determining "how" to fix the problem -- and it very well may take an experienced dog trainer to understand what your dog is thinking.
However, leash aggression is typically seen in a dog when he is on a leash--with the growling and lunging being directed specifically at another dog. If your dog is showing any aggression towards people, then this is not classic leash aggression, and your dog needs immediate help from a qualified trainer.
Most dogs that display leash aggression are actually nice at all other times. There is something about the combination of the leash and the owner and an oncoming dog that seems to trigger the behavior. In fact, those three ingredients must be present for 'classic' leash aggression.
In spite of a leash aggressive dog's outward appearance, this misbehavior is often rooted in a dog's fear of the unknown. A fearful or over-protective dog has two options when confronted with a scary situation--run away or get ready to fight. Since running away isn't an option (he's on a leash that you're holding, after all), then posturing as though he is ready to fight is the only choice left.
Most leash aggressive dogs don't have complete confidence that their owner is in control of the situation, and so the dog seeks to take control by lunging and growling. Even though the dog seems to be saying "I dare you to come over here," what he really wants to say is "go away, you're worrying me."
In general, most leash aggressive dogs don't recognize their owner's authority and top dog status. You can become top dog again by taking control when you are going for a walk. Don't let your dog walk in front of you--this is reserved for the top dog. Instead, make your dog heel at your side and adjust his direction and pace to your desires. If your dog is too powerful to control then use a Gentle Leader collar or a martingale collar. The Gentle Leader tightens over the nose and turns your dog back toward you, and the martingale tightens over the throat. Both collars give your dog feedback that he is at the end of his leash and needs to come back to his top dog's side.
If you are using a harness on your leash aggressive dog, then throw it away. Harnesses are excellent for dogs pulling sleds, but not at all effective at reinforcing your top dog status or controlling a lunging dog.
Now that your dog is walking alongside you (isn't that new collar nice?), it is time to start reminding him of some basic obedience training. Your dog should follow you, turn when you turn, and change speed as you do. I recommend quickly changing directions from time to time, to make sure that the dog is responding to you. The collar will give your dog a 'correction' if he doesn't follow you.
Having taken charge and reasserted yourself as leader of the pack, now you are ready to deal with the dog's leash aggression. The key is for the dog to focus on his owner, and not on some other dog that is approaching from the opposite direction. If you spot another dog along the path, then start giving your dog commands and instructions to follow. Change directions and reinforce your discipline.
Once the dog realizes that you are in control, he is a lot less likely to be fearful or overly protective about another dog that is approaching. After all, you are the top dog and you've taken charge of the situation. Now that he is confident in your leadership, the two of you can get back to having enjoyable walks again.
Dr. Jon Klingborg is a veterinarian at Valley Animal Hospital in Merced. He can be reached at email@example.com.