This is a hard thing to correct, but I found that if I notice the other dog before my dog does, I can have him sit/stay and reward him for that. I have trained him that if he sits and stays without barking, he gets to meet the dog. It takes a lot of time and patience and one very helpful thing I've found is taking him to the dog park and walking around the perimeter. Once he stops pulling and barking, he gets to go in. You can also train him in this way by having a friend meet you with another dog. Make sure its one hour dig won't recognize so you can start training him to react differently once he meets new dogs.