I've kept a giant cardboard box of notebooks and papers for the past fifteen years. There's always something in the mulch that I can pull out and revisit. It takes me quite a long time to get an idea ready to write, but intermittently playing with ideas and story structures keeps me excited and inspired no matter what stage a book is in.
If I'm fishing for a brand-new idea, I put together a targeted Book Stack and chew through it, taking notes as I go. My best ideas are mutations of ideas I've read elsewhere.