Item zero: always be working on something, if only to be keeping your mental state creative and healthy. To me, that can be a book project as well as it could be fixing a dam, raising a kid, washing your laundry or whatever. This may sound a little Ben Franklin, but I think the mind works best that is steered towards well-considered goals.
In our case, it means always be holding an image or an idea. Maybe that image is a character, maybe that idea is an opinion or plot point, but keep it and roll it around in your pocket or in some part of your brain.
Tumble it with your time and attention. Now it’s a piece of glass, a stone being polished. Whether you are full of random thoughts that are going nowhere, or if you are actively engaged, apply your thoughts and ideas to this held image or idea, test them out, build your store, and develop your project.