You need to write something about which you’re passionate, something that gets you up in the morning, draws you to the keyboard, and keeps you there. It should excite not only you, but also anyone you tell about it.
How do you know you’ve got a winner? Does it have legs? In other words, does it stay in your mind, growing and developing every time you think of it?
The right concept simply works, and you’ll know it when you land on it. Most importantly, your idea must capture you in such a way that you’re compelled to write it. Otherwise you’ll lose interest halfway through and never finish.