Here's some good advice: if you want to accomplish something, you need to actually set aside time in which to do it. Blatantly obvious though this is, it's frustratingly hard to put into practice.
This is my take on the subject: don't give yourself a choice. Treat as something you just have to do. In this spirit, I have now set aside Monday as "writing day". Every Monday, no matter what, I will sit down on work on my book. No messing about, no distractions, just do it.
Yesterday was Monday. My first writing day. It went well. I spent probably 3-4 hours just writing and researching, and though I'm still on the ever-expanding introduction (or maybe it's chapter one, it's too soon to tell) I probably got a good three pages written. That feels like a good day to me.
I'm wondering if it might also help if I released the book a chapter at a time, perhaps to interested friends or regular readers of this blog (I know there's at least a few of you). That might stop me endlessly re-writing it, a trap I've fallen into in the past. It would also be a good way to get feedback from kind and intelligent people (oh how I flatter you) before unleashing the eventual monstrosity on the wider world. What do you think? Please comment below. It's about veganism, transhumanism, evolution and that sort of thing. Working title: 'The Vegan Imperative'. Working subtitle: 'Animals, humans and the future of life'. Grand.