Some days the words just don't want to flow. Every idea I come up with seems dull, stupid, and unworkable. When that happens, I often fall back on the "write 100 words" plan. I focus on writing 100 words even if they're not the best, even if I ultimately end up throwing them away. It's an attempt to loosen the writing muscles and it works surprisingly well. It clears the cobwebs out, erases some of the dreaded fear that all writers face, and it spawns more words (at the very least).
Other days, when I've yet to come up with a workable idea for a book, I focus on brainstorming 10 quick ideas, just scribbling down the basics. None of those ideas may end up being a gem, but if I take the best one and begin drilling down, I'll often find that a new, workable idea will spring forth from that process.
Not every writing day is a golden one, but on each and every writing day, one thing has to happen. I have to write. Because there is one undeniable fact, and that is, if I write zero words, I'll never create even one more book. And every day away from the writing makes it that much more difficult to write. Writing muscles, like physical muscles, need to be used and exercised if they are to stay in top form.