A cohesive voice means that all parts are complimentary. You must have energy, authority, and trust. Even if the pace of the story is a bit slow, the narrator needs to be engaged and interested in the story—energetic. The narrator needs to be clear and assertive (we have to trust that the narrator is an authority of the story and can tell it well).
Even in a work where the narrator isn't trustworthy, such as in The Thief, it is essential that we still trust him to tell the story well. We don't have to know all the details to trust the narrator, we just have to trust his story-telling ability.
An authentic voice is realistic writing from the heart. Writers tend to emulate other writers. Don't try to copy Harry Potter or Twilight; don't write to a trend. Write a story the way you hear the story. Check the sound of your story, and make sure that it sounds right.
Mihalick used a quote by Emma Bull to illustrate this:
A book makes friends with people.Readers want to become friends with the storyteller. We seek the voice of the storyteller and strive to have the sort of intimacy with a book as we have with our friends. The premise of Harry Potter wasn't knew—the voice was. Likewise, Twilight is nothing more than a forbidden love story, but the voice invited new readers to that old story. When editors say they want something that sounds fresh, that's what they mean: a voice that is fresh and new. A good voice will make an editor forget other similar stories.
You know you've got a good voice when you can make readers (editors) hear that voice in their heads, think in that voice, consider what a character in that voice would do in a situation.
So how do you learn how to have a good voice? You don't. You "can't teach someone to have a voice; a voice can only be found." Her suggestion is to listen. Listen to others, listen to the way things are written. Find your favorite passages in books and analyze what made them so memorable in your mind. In the case of children's books, also listen to children's voices—how they talk and what they talk about—and integrate that into your writing.