Some writers, such as James Patterson, outline every inch of their book before starting in on the actual writing. While an outline can be a useful tool, it can also limit your ability to utilize good ideas later on. By design it creates a box for you to work in. But while that box provides direction, it can also back you into a corner. You may come up with a great idea once you’ve gotten started that you can’t figure a way to fit inside your less interesting, earlier plan.
To avoid the need to constantly revise an outline, I prefer a more organic approach – starting out with a broad sense of where the story starts and is headed, and then filling in the details as they begin to present themselves. This allows for some flexibility while also supplying a general direction. It’s about striking a balance between planning and discovery.
In my experience, a lot of the best ideas come spontaneously. It’s important to leave room to allow that to happen.