Once again obedience training is the key. At the first sign of any anxious or aggressive behavior from your dog you can immediately call on an obedience command such as a down-stay to divert his/her attention. You are asking your dog to perform an alternate behavior which takes his focus and attention away from the other dog. It also changes your dogs body language to a passive, non threatening posture.
When you are in the process of eradicating on leash aggression be sure to use a suitable muzzle and do your best to avoid possible confrontations. This won't fix the problem but it's a worthwhile temporary measure.
Always be mindful that your dog is very sensitive to your energy, emotions, breathing and feelings. Therefore if you tense up and grab hold of the leash tightly at the first sign of an approaching dog, your dog will pick up on this and become anxious and stressed. This is a huge factor in most cases of on leash aggression.
You want your dog to believe that other dogs are no big deal rather than something to get worked up about. Another reason to not tighten up the leash is because this changes your dog's body language (makes your dog stand upright and tall). This can be seen by the other dogs as a show of dominance or at the very least threatening.
Teach your dog the obedience "look" command or "focused attention exercise". When taught correctly this exercise can be called upon anytime you require your dog to focus on you and off something else - such as an unfamiliar dog. Follow these steps:
As with teaching any new command start in a familiar environment to your dog, free from any distractions (don't start teaching this attention exercise when you are out and about on your walk).
This exercise is all about getting and holding the attention of your dog, so grab a handful of your dogs favorite treats and lets get started!
Step 1: With your dog on leash say "Fido" (your dogs name) "look", as soon as your dog looks up at you (gaining eye contact) praise him/her and then produce the treat from your pocket and give it. Remember to keep this sequence the same every time "Fido" look!, as soon as you gain eye contact immediately praise your dog "good boy!", then provide the treat.
Step 2 is the foundation of the exercise, get it right and then you just need to build on it and strengthen it. Add some variables to this basic exercise such as saying "Fido look!" then take a couple of steps to one side. When your dog follows you and looks up to make eye contact you praise and produce the yummy treat. Now you can lengthen the amount of time you have your dog's attention by repeating this exercise back to back. It goes like this, say "Fido look!" take a couple of steps to your right, your dog follows you and looks up into your eyes, you praise and then treat. Straight away you repeat this process (step to the left this time) and continue to do it 5 or 6 times.
Keep practicing this exercise over and over and take it to different locations and gradually add some distractions such as the presence of other dogs. This may take a while, take it slow!
When you've built a reliable "look" command in any environment, you can confidently call on it in many situations, including when other dogs are around. Eventually you will be able to fade out the treats and just rely on praise and maybe an occasional treat. In the end you'll find your dog will look to you whenever other dogs are around. Your dog will soon learn that there is no need to be anxious or to fear other dogs. You'll find that eventually your dog will actually learn to associate the presence of other dogs with something positive happening.
Another technique is to play the "find it" game. This redirects your dog's attention, breaks eye contact with any other dogs and produces non threatening body language from your dog. All you need to do is throw a treat on the ground and say "find it". Your dog will pick this game up very quickly and is sure to love it.
Always reward your dog for polite, calm greetings with unfamiliar dogs. Demonstrate to your dog that you are happy with him/her.
