Pack Instinct
A domesticated dog still operates from the perspective of pack instinct, and understanding this will help you learn the most effective ways to introduce a new dog in the house. Pack instinct refers to the natural and characteristic ways that dogs respond in any relational context. This includes relating to other dogs as well as other people. Even though your dog is aware that you are not another dog, he will relate to you according to pack instinct. It's all he knows. Thus, you have to communicate with him in a way that speaks to that instinct.
Dominance and Pack Instinct
In every canine pack, there's a dominant dog. When a dog first comes into your home, he will most likely assert himself as the pack leader until he realizes that you or another dog are the pack leader. For the safety of everyone involved, a human being should be the pack leader in every household with a dog. You can teach the dog that all humans are pack leaders, but especially everyone who lives in the house.
Rank Drive
Some dogs will attempt to become the pack leader even when another pack leader is established. Some dogs will only attempt to usurp the ranking of another dog while respecting the authority of human beings. Others will challenge the dominance and authority of the human who is in charge, which creates a special challenge when introducing the dog into the house. The severity of rank drive varies greatly from the dog who just sees what he can get away with, all the way up to the dog who will attack his owner. A dog with rank drive isn't the best choice for someone who has never raised a dog before. Once a dog with rank drive is trained, he is a natural leader in social and work situations because he gets to show off as the best.
Asserting Yourself as the Pack Leader
Introducing a dog into the house requires a combination of love, patience and assertiveness. You can establish yourself as the pack leader while creating and maintaining a very loving and fun relationship with your dog. Dogs actually feel more anxious when they don't know who the pack leader is.
When you first bring a new dog into the house, establish boundaries and rules that you consistently adhere to. Create a reward-based system for everything so that the dog learns you're in charge. If he goes to his bed, he gets a treat. If he's on the couch, he'll think he's equal to you. If you calmly and quickly (but sternly) tell him no, lead him to his bed, and give him a treat once he's on his bed, he'll learn that you are a loving, yet clear leader.
Using short abrasive sounds and firm yet non-punitive nudges away from forbidden places or actions, mimics pack leader behavior in the wild. Your dog will feel safe and secure. You can still play with him and be affectionate with him but it's on your terms, not his.