I've been experimenting with different methods for working on this, much longer than I thought it would be, writing prompts book. My current method involves getting up early to write the first sentence or two for as many prompts as I can think of. I then work on some of my freelance stuff and later come back to fill out each prompt.
Since coming up with the initial idea is typically the toughest part, getting that out of the way before I'm completely awake pushes through writer's block later on. Twenty-four of the prompts pages are now on my website along with a post I wrote today about writing motivation.