Find your voice, your time, and your place. Writing is the disciplined art of managing your own mind in order to communicate forcefully for hours at a time. Every writer needs to know when and where he or she writes best. Often when I'm writing, I tell no one where I am. I shut off the phone and I ignore email. Distractions are murder on writing.
I recommend keeping two book files - the book itself and a slush file to which you can cut and paste what's not working. Writing is as much cutting and revising as it is the initial creation. It's much easier to put a passage that's not working in the slush file than to chop it altogether.
If you're writing nonfiction, don't touch your endnotes until the end. Copy a URL or make a quick endnote to remind yourself where you got a particular fact. Then keep writing. If you invest to much in the sourcing too early, you'll be less likely to cut or revise when you should.
Let everything you write sit for at least a few days, return to it fresh, then be merciless on your own stuff. If you're like me, you'll find sections that just need a little polishing and sections that need to go to the slush file. That repeated filtering means everything to creating a well organized and well written book.