I find that it saves time to plan the plot of the entire novel (or the entire series of novels) before I begin writing it. I usually just keep the plan in my head and only write down a list of chapter names so I can remind myself what the plan was. I find that if I write down the entire plan, in detail, I end up writing a very stiff sort of prose when I try to write the actual novel. With no plan at all, I have to re-write a lot of text because the plot keeps taking wrong turns, it takes too long to get to the ending that I'm aiming for, or I just need to go back and insert little bits of foreshadowing into older chapters. With a vague idea of the plot and the names of the chapters to remind me a little bit about what happens in each chapter, the writing goes smoothly and the story stays focused on the main plot.
One problem that I used to have is keeping the same "voice" throughout the entire novel. My solution for that is to write in a voice that's easy for me to keep using, something that feels natural, even when I'm tempted to use a voice that would feel more in-character (such as a voice that I'd associate with a certain century or a first-person narrative from the point of view of a very particular sort of character).
I have a terrible memory when it comes to names and dates, so I keep a list of character names, job titles, names of towns and of other locations, and other things that I might forget. I think it would also help me if I kept a list of details to go with each name (physical appearance of the characters, and any important details about them that I might have written in a previous chapter) so that I don't run into continuity problems, but I usually just go back to the older chapters where that character appeared, and re-read the entire chapter to remind myself of everything I need to know.
Speaking of re-reading, I think proofreading and editing are very important, and I think it's best to do this a few months after you finish writing the novel, when you don't remember the novel all that well anymore. I have a particularly bad memory, so that helps me get a fresh perspective on my own work, as long as I let it sit there long enough.