Shawn Blanc shares a handy writing productivity tip, amongst other insights into his process:
500 words ago, I lied to you. I said my writing begins at 7:30 every morning.
The truth is that my writing for this morning began yesterday when I put that note on my desk. That note is my topic for the day. That note is the single most important element of my personal productivity system. Because that note is the single most important thing I have to do today.
In a sense I'm reminded of Ernest Hemingway's method (scroll to page 6 of that PDF) of keeping the writing momentum going from day to day:
The best way is always to stop when you are going good and when you know what will happen next. [...] Always stop when you are going good and donʼt think about it or worry about it until you start to write the next day. That way your subconscious will work on it all the time. But if you think about it consciously or worry about it you will kill it and your brain will be tired before you start.