Bringing a new dog into a home with an existing dog can be a challenge, as can introducing your pup to a friend's dog. But there are a few techniques that can make the introductions become more like handshakes than dominance attacks. Start out slowly, letting them get to know each other a bit before expecting them to play well together.
Smell First
Learning each other's scents helps the dogs become accustomed to each other before they ever meet. Especially if the dogs will be living together, introducing the scent of the other dog before they meet can help the transition go smoothly. Try placing blankets or soft toys in the crates of both dogs, making sure the dogs can't see each other by placing them in separate rooms. After a day, switch the toys and blankets so the dogs smell each other. Give them a day or so to learn the new smell before letting them see each other.
Barrier
If you're not quite sure how the dogs will get along, try introducing them with a barrier in between. A chain-link fence is ideal, giving you a clear definition of separation between the pooches while letting them see, smell and interact with each other. A wooden fence with openings between the slats might also work, as can a large baby gate if you're dealing with smaller dogs. Let them meet a couple of times through the fence before bringing them together without a barrier.
Neutral Location
Dogs are often territorial, not taking kindly to new dogs who invade. This makes a neutral meeting location key to a successful introduction. Take the dogs to a park or a neighbor's yard, then walk them on leashes across from each other with a gap of at least 20 feet. They can still smell and see each other, but neither is considered a threat from that distance. Gradually walk them closer together until they can touch noses and start introducing themselves. Use the leashes to separate the dogs if one starts acting aggressively.
Let the Play Begin
After they've met for a few minutes with no aggression showing in either dog, it's time to unleash the hounds and let them get on with the business of playing. Dogs relate to each other through play, so expect them to bark, nip, wrestle and chase in a playful manner. Be prepared to step in and grab collars to separate the dogs if one starts growling or biting. End the play after just a little while, such as half an hour, to keep the dogs from losing patience with the play date. Introduce them again the next day for a longer period, and spend another few days bringing the dogs together before moving a new one into your house or having them spend time together without your oversight.