"We should take care not to make intellect our god; it has, of course, powerful muscles, but no personality."
As a general rule I define Originality as "that spinning out of an idea that is not commonly seen, and which sprang upon you, rather than you springing upon it." You know when you've hit something original, because it will probably have very little to do with anything else you've ever read. Sometimes it will resemble in some way another author's work because after all, there are only so many basic plots. BUT--an original idea won't be concocted of bits of Jane Austen's novels smashed together and called your own. Publishers don't need more conglomerations of what they've already seen. They need an idea that stands out of the hoards because of the fact that it is exotic and rare. Nobody, walking through a park, would stop to look at pigeons when there was a swan floating on the lake. You want your novel to be that swan--that one book that catches the eye of the Lofty Ones so that they take a second glance.
So how do you make sure your work smacks of originality? By avoiding a host of things, some of which I've listed below for your benefit and mine.
Stop Using the "Different Girl"-- Rookie Mistake No. 1 is making your protagonist a willful, headstrong, freedom-grasping girl who only wants to "be different" and spends the whole novel fighting with tooth and claw to prove herself. If I have suddenly squashed your little character-bubble, forgive me. But instead of wasting a whole novel saying your character's different, why not make her different from the start? After all--being different is all a part of that self-same Originality. But show it--don't tell it. We are tired of the Different Girl--and her horse. Which brings me to Point Two:
You Don't Have to Include a Horse: Not that there's anything wrong with a horse, but honestly--if you tallied up all the books that have ever been published, I assure you that books with a strong horse-character are already crowding neck and neck with romance to claim Most Used Ploy. Try something new.
Let your voice play out: Instead of trying to be original, just write as fast and furious as you can. Your natural voice will develop and mature in this way, and a good natural voice is another thing publishers look for. If you are trying too hard your writing will feel stilted. Just write and let it lie. Tighten things that need tightening, and cultivate your craft, but always keep your voice intact.
Pay attention: The thing is, there are thousands of stories crowding around us every day in the form of People, yet because we are so absorbed in our own troubles, our own business and hey--even our own imaginations--we fail to gather anything from this treasury. Go out, get coffee or a doughnut or something and just watch people. If you are in an airport and waiting for a flight, don't grab your book. Sit there and watch life go by, taking stock of things for once.
Keep a notebook handy: Anytime an interaction, description, or event pops into your mind, go to that notebook and catch the idea on paper. You don't have to have any definitive plans for the pieces you catch, but I can assure you that they'll be helpful. You see, originality largely depends on the expanse of your mind. If you can turn your notebook into a second mind for yourself that can remember things you would have forgotten, then so much the better!
"Keep it secret; keep it safe": I recently heard another author speaking about the fact that your characters need secrets. Yes, you know the character well, but just as you don't know everything about your friends to begin with, so your characters should reflect that maxim. Keep your readers guessing. Let your characters have motives and secrets they might keep hidden even from you for a time, only to let them fly at a crucial moment in your story.
These are my tips and things that I have found helpful in cultivating a sense of originality in my writing. I hope they'll help you, and that someday I'll get to read dozens of original stories pouring out of publishing houses as we all strive to have fresh, new ideas! :)