One of the most important lessons any dog should learn is to wait - to wait for everything and anything you are looking to provide for him.
Whether it’s food, water, a toy, your touch, opening the door, putting the leash on, and so on, your dog is ready to wait if you simply expect him to and in turn wait while he figures out that you will reward his patience and trust.
We often mistake a dog sitting down, as one that is waiting calmly. And even worse, the amount of time we expect our dog to remain in a seated position is usually no more than a couple of seconds. We need to teach your dogs to wait for as long as we require them to. This is how a dog can learn to no longer demand things but to instead trust and wait that you will let him know when it is that he will get something from you!
This is the equivalent of giving your dog a kong and letting him figure out how to get the treat out. In this case, however, the treat is whatever you'd like to provide your dog with. Let your dog try barking at you, perhaps even howling or jumping. But the moment you obtain calmness after having said 'Sit', is the moment you can nurture that state of mind with the act of giving something to your dog. Then the next time, have him wait for longer. This is where the challenge lies for your dog, but the bigger the difficulty, the more rewarding it is once your dog figures out that calmness, relaxation, peace, and trust is the key to everything he has ever wanted.
Always remember that waiting is something they are born knowing how to do. During puppyhood, the mother will indicate when feeding time begins and when it ends. She doesn't feed the pups because they ask her to, nor because it's a particular time or hour of the day that she follows. She simply decides when the right time to feed the puppies is, and then lets them know.
This video is a good example of this: https://www.youtube.com/watch?v=KHBe0jT6S3U
In the video, not only is the mother of the pups making them wait, but she is also making sure that they're waiting calmly, above all. This is something we simply do not practice at home, once we bring home our puppy, but we really should!