There's a common misunderstanding in many people's minds about dogs and "dominance." Unfortunately, TV stars and other mass media have perpetuated a myth that dogs are "dominant" and trying to exert control over their human families. Rest assured, it's simply not true that your dog is trying to take over and that you need to show him who's boss. Your dog knows you are the boss.
Your dog's behaviour is an attempt to meet his needs, usually the need for safety and security, distance and control. In this context, control does not mean that he wants to take over and be alpha. Control means that your dog is trying to exert an effect on the environment. For example, if he is scared, he will bark and lunge and growl because he wants the scary thing to go away. Our job is to teach him gentler, more acceptable ways to meet his need to feel safe.