You know that show with Christina Aguilara, CeeLo Green, Adam Levine, and that other guy? What was it called? Oh yeah-
I've been following a lot of agents and editors, perusing their blogs, inhaling their tweets, studying their clients, etc. And I have discovered that there are certain qualities of writing that they are always on the lookout for, and others that they don't worry about as much. You'd be surprised at how often writers are worried about things (word count, grammatical minutia) that agents shrug off, as long as the more important elements are there.
A smattering of examples would be: a riveting plot, tenacious characters, unforgettable dialogue, captivating setting, and that element that is so confusing for authors, but may be the most important element of writing, the unique, engaging voice.
So, what is "the voice"? Putting it simply, it is the "you" in your writing. If plot, characters, dialogue, and setting are the building blocks of the story, voice is the substance of the telling. (story+telling. You got that, right?)
Even though there are a limited number of stories out there, and the same story has been told a thousand times, it's the voice of the writer that sets each one apart. How can you read a piece and know that it's Mark Twain, or H.G. Wells. Not because they tell different stories, but because their stories are told in different ways.
So, how do you develop your voice? I think it begins with exposing yourself to as many authors as possible.
To clarify, I'm not endorsing flashing your favorite writers. (no matter how many of them are hoping for that advisement.) Rather, I mean you have to read. And read. And read some more.
But don't read for prestige. In other words, don't read because you need to read certain books to be "well read" or something. Instead, read to hear the voice of the books. Don't worry about reading all of the books, really, but focus on reading just the first chapter. If the voice doesn't capture you, move on. If it does, read on.
Once you've read and read and read, start writing. Not your book, though. Sure, your book is important, but try writing something more personal. Recount an event from your life, and try to really get your voice in place. Maybe do it a few times, emphasizing different aspects of your voice. Remember, this isn't for publishing, not even for anyone else to see. This is just for you to experiment.
Once you've started really finding your voice, it's time to go one more step. Now you have to learn to write in a voice other than your personal one. That way you can write the mysteries, the romances, the kid-lit, etc. How do you do that? Try acting classes. Seriously. Learn how to embody another character, learn how to make someone else's voice your own.
Then you can put yourself out there. You can write with confidence, knowing that you can maintain an authentic voice no matter what it is you're writing.
And your stories will be that much better.