It's not enough to have a great idea that's really interesting. It's not enough to plan carefully, and work out a perfectly precise outline. It's not even enough to create highly engaging characters. You've got to really sink into the story, until it comes so naturally you're practically speaking in an accent to your family members.
Drowning In It
Here's an example of what I mean: my newest book is about a girl who's always lived her life by the ocean. I have not. But in the book, I used quite a few different metaphors and phrases that are ocean-related. But I didn't have to think about using them or struggle to put them in. That's what being in the zone means, I think. When you're really deep into the story, things that fit into it just start coming to you naturally.
At least, that's the way it works for me. But there are a lot of ways to encourage that process, so if it hasn't ever happened to you that way don't be discouraged. You can begin to immerse yourself in the world you're creating even before you start to write it.
I'm a huge advocate of doing research and getting a strong background before you begin writing, though inevitably you're going to have to stop writing frequently to look up new information. When you research your topic thoroughly, you can start to picture it even before you write it. You can begin to see it in your mind, and think about what it's like to stand there. You're putting yourself into that environment already. Once you actually begin writing, then, you'll already have a connection to that world. That will make the writing of it easier.
If you keep visualizing your world and thinking about it, and if you have a solid understanding of that world, you'll be able to write like you're in that world...because you are.