Don't write your entire game before the players make their characters.

Take their backstories, and look for elements that seem interesting or key. Work them into your plot. This can be either by taking their things and replacing things you already have (oh, I've got a dangerous cult, but they want the Followers of Night stalking them, so I'll use that instead) or by finding a way to insert their ideas into your plot/story (oh, they want the Followers of Night stalking them? Cool, I can use them as a puppet of Mr. Evil).

Think of it like Iron Chef, only with GMing.