To stop your dog from barking at other dogs, help him learn to be comfortable around other dogs and teach a different response. This process often moves slowly but can build a stronger relationship between you and your dog while helping him develop more confidence.
Positive Reinforcement
Many dogs who react to other dogs are barking because they are frightened. Some are barking because they actually want to play with other dogs and are frustrated. The dog who barks because he just hates other dogs is rare.
No matter the reason, punishing a dog for barking is not going to solve the problem. In fact, it will likely make it worse. You start correcting earlier and more often, and most owners don't use consistency in giving corrections. Thus, every time your dog sees another dog, he knows something bad is about to happen. This will cause the barking to get worse because your dog wants the other dog to go away so he won't get corrected. He doesn't realize it's his behavior causing the correction.
Instead, teach your dog an alternative behavior and reward for that. Rewards can be food or toys, depending on which motivates your dog more. At first, it should be your dog's favorite treat (like steak) or toy (squirrel hide) and then you gradually wean your dog off the reinforcers and use more praise. Remember to always praise before giving the reward, so the praise becomes as rewarding as the treat.
Create a Positive Association
If your dog is fearful, you want to first create a positive association toward other dogs. Do this by taking your dog somewhere that he might see another dog. Stand far enough away from the other dogs that your pet notices them but does not react. This may be very far away at first. Reward your dog for looking at the other dogs and not reacting. Give the best treats you have.
Gradually begin to move closer. This will take time. If your dog ever reacts, you've moved too quickly. Keep these sessions short so they always end on a good note. The longer the session lasts, the more likely your dog will react. Start with a minute or two and build up to no longer than 10 minutes per session.
Teach an Alternative Behavior
Think about what you would like your dog to do instead of bark, for example, walk politely by your side. Now, you have to teach your dog to do that.
Start in the most quiet room of your house. Hand feed your dog for at least a month. All of his meals are earned by practicing this behavior. Say "Look" or "Watch." If your dog looks at you, he gets a handful of food. Lure him into the heel position and repeat. Practice until your dog can heel and look at you immediately.
Then move to your back yard. Practice until it's perfect. Move to your front yard. Gradually, make the situation more distracting. Take a few steps before rewarding. Build up to the point where your dog can look at you rather than another dog.
Though this takes practice, these methods are easy for a dog to learn and understand. By creating a positive association and teaching alternative behaviors, you're teaching your dog how to relax and be more confident.