Outlines. Writing a book takes organization. I've written a novel, it's about 90,000 words or so, and it took me a little over a year. I had a plot outline, drew maps of locations to make sure geography and stuff made sense, wrote essays describing the characters and their motivations, what they knew and what they didn't know, lists of the stuff they had on them, had background stories for everybody, including shit like pet dogs they had as children and their favorite pair of shoes. You get the idea. I know so much ridiculous shit about everyone and everything in the story, and about 90% never actually gets related, but helps to flavor what DOES go on paper.
Writing a novel is fucking WORK. It's not going to happen if you don't have a clear idea about what you want to say, the story you want to tell and a game plan to go about it. After many abortive attempts at story writing I finally figured that out, and slogged through word after word, doing countless rewrites and edits, outline not far from hand.
My novel may or may not be worth a shit, I don't know. But it's fucking finished. I learned a ton about writing in the doing of this thing, which may have been the most important thing.