Write notes about your book into something like Evernote, which can be accessed from your phone, your desktop, and any web browser (meaning you have no excuses to take down ideas).
Keep 3×5 index cards in your pocket or bag, and jot notes there, too. A lot of writing is done before you sit down to actually write.
Build loose-but-useful outlines and seek out 20 minutes here and there to “shade them in.” People facing a blank page waste too much time thinking about the page. Instead, work on bits that need work.
If you work better speaking, look into a product like Dragon Naturally Speaking to do dictation for you.
Prone to distractions? Try Ommwriter for Mac (or PC).
Also, shutting off the Internet helps.