Remember, dogs do not speak English, therefore you must SHOW your dog what you want. Your actions speak louder than your words. All of your body language speaks to your dog. Therefore, your ATTITUDE, FACIAL EXPRESSION AND TONE OF VOICE communicate to your dog. You cannot try and tell your dog that he is not doing the right thing while you are hiding a laugh because you really think your dog's behavior is cute or funny. Your dog will laugh right along with you. You cannot ask your dog to obey you if you hesitate in your movements. Your dog will not believe that you are the leader. On the other hand, you cannot bully your dog or physically punish him and expect your dog to respect and trust you. A good working relationship with your dog is built on trust and leadership. This is communicated to your dog by giving commands in a tone of voice that says, "I expect you to do this, no discussion." Then move in a steady, yet gentle way to convey leadership.
Never re-command your dog. If your dog knows what the word means, re-commanding him just teaches your dog that a) he does not have to listen to you; b)he can do it when he wants and c)you are not the leader. For every command there should be an action. Either you coax the dog into doing what you want or the dog does it. If your dog does not know the "sit" command, and you tell your dog, "Sit . . . Sit . . . SIT!", then make the dog sit on the third sit, your dog will learn not to sit until the third command. He will think that the command is "sitsitsit."
Most people repeat commands to their dog's because they are being polite (according to human standards) and assume that the dog did not hear the first or second time. I can assure you that if your dog does not respond the fist time, and does not acknowledge you, he DOES HEAR you. He is just IGNORING you. Therefore, politeness to a dog translates into "My owner is wimpy, wimpy, wimpy! Why should I listen?"
So, speak clearly and in a direct manner to your dog. For every command expect or initiate an action. Do not repeat commands. Do not hesitate when you move. Show your dog that you are a leader, not a follower.