Here is a tip from one of our PetPlace.com users:
I am a foster home for my local animal shelter so I get a “new” dog every other month. I have 2 dogs of my own as well. One foster I took in a while back was a very sweet girl, very loving and very playful. But for some reason she was very afraid of other dogs and would often growl at my dogs. My dogs would smell her and stay very close to her, like she was invading their grounds (even though this was nothing new to my dogs–they just weren’t sure about this dog).
I don’t remember why I did it but I had to take this dog to the vet and couldn’t find an extra collar so I took the one off of one of my dogs for the foster dog to wear. Anyway, when we got home from wherever we went I took her into the yard. My dogs smelled her, smelled the collar and then walked away. My longhaired dog now wears two collars and when I go to pick up a new foster from the shelter I take one of them off and use it on the foster I bring home. By the time we get home she has the scent of my other dog, almost the “I belong” smell and it makes introductions a lot easier. It adds a bit of comfort to me when bringing in a new dog.