2. Thou shalt know how it ends
This may seem obvious, but based on numerous books I have read, knowing the ending is something that far too few writers do before they initially set their metaphorical pen to paper.
There are three types of novelists. The first is the Outliner. These are highly organized writers who are able to carefully plan out how the book will proceed and more or less stick to their plan. This is probably the ideal way to go about writing novels, but it’s also extremely difficult if you are insufficiently organized. Outliners tend to write books that are tightly plotted, idea-driven, complex, and formulaic. Due to the complexity and scope of A Throne of Bones, most people assume that I am an Outliner, but as it happens, I am not. JK Rowling is one example of an outliner and I suspect most writers of murder mystery series are based on the predictable sequence of events in many murder mysteries.
The second type of novelist is the Explorer. Most authors are Explorers and not only don’t have an outline to hand, they often have no idea what they’re going to write about when they sit down and stare at the blank page. They tend to follow the story where it takes them rather than forcing the story into preconceived directions. Explorers tend to write books that start well and finish badly, (or vice-versa), that are character-driven, dialogue-heavy, and of varying quality from book to book. I am an Explorer; of all the various characters who died in ATOB, there were only two characters whose deaths were planned and one of them was dictated by the historical event upon which the situation was based.
The third type of novelist is the Autobiographer. This is the author who is the protagonist of his every book. They are generally uninterested in anything that isn’t themselves; if one looks closely enough, one can always see the image of the author underneath all of the major characters. If their lives or personalities are sufficiently interesting, Autobiographers may have one or two very good books in them, after which point they run out of material as their books are experience-driven rather than plot- or idea-driven. Jay McInerney is the foremost example of an Autobiographer; Dave Eggers and David Foster Wallace are more recent examples. I also suspect that McRapey is an Autobiographer, which explains why his novels are more akin to professional fan fiction than original fiction.
But regardless of what type of novelist you are, it is always vital to know exactly how the book ends. When I started writing ATOB, I knew precisely three things: the prologue, the conflict between the general and his young tribune, and the long retreat north. But because I knew the beginning and the end, I had the necessary anchor points to prevent the story from wandering aimlessly adrift.
In the absence of an outline, knowing the end helps pace the story and forces it to keep moving forward. This can be done well or it can be done poorly, but one way or another, it will be done. We have all read authors who wait too long to begin the descent into the end and wind up accelerating the story and crashing the book in the last two or three chapters, but even these negative examples are better than books that simply seem to stop without any warning or reason. Knowing the end won’t only make the ending better, but it will help make everything in between the beginning and the end much more coherent.