Have more than one idea on the go at any one time. If it's a choice between writing a book and doing nothing I will always choose the latter. It's only if I have an idea for two books that I choose one rather than the other. ~Geoff Dyer
Mr. Dyer is entirely correct. Any writer that hopes to be more than a one-hit-wonder must always, always have at the ready a rich reserve of ideas, plots, characters.
I had this conversation with my mother just yesterday -- there's a specific sort of panic that sets in each time I finish a novel. It's the irrational feeling that I'll never write another book again. That the well has forever run dry, that I've stopped knowing how to write, that the Muse has abandoned me and will never return. See? Irrational. But very real.
One powerful antidote to counteract that sort of panic is to have an ongoing, eternal brainstorming file. Whether kept on your hard drive or in a notebook, every writer should have one. That way, the well won't ever run dry.
Thankfully, I'm in the middle of a book series that has a long end in sight. I've just finished Book 3, and could probably easily go up to Book 10, if I so desire. I have familiar characters and a familiar setting, with stories still to be told.
But even so, I'm not settling into my comfort zone. Only yesterday, I started getting ideas for yet another possible series - a young adult series along the lines of Anne of Green Gables. So, rather than let those ideas flit into and out of my mind, I sat down and added them to my "brainstorming file." With as much detail as I could, I wrote about 2 pages of future ideas, for characters and plots and themes. I might not even see those ideas come to fruition for a few years, but at least I know they're there. And that's extremely comforting.