Unique

I had one of those dreams last night, that made me think Thats a great idea for a story, but in true dreamlike fashion the more I try to remember and write down the more it slips away and what I do remember just seems ridiculous!
I find that sometimes the ideas that do seem like they could hold someone's attention for longer than a few minutes, start to be second guessed because it feels like they been done before.
In world filled with thousands of books, how do you come up with something that's completely unique? How do you create something that will push to the front of the crowd and not fade in the background?