Keep a notebook.
I heard that writing advice early on. I don’t remember where, probably many places. And it is such excellent advice, I expect you can find it everywhere. These days I use the notes APP on my iPhone because it’s the only thing I can be sure to have on me at all time. But an old fashioned notebook would work well too. Sometimes, I write down something I saw or overheard. Sometimes it’s an image. But anytime I think of anything I write it down. Sometimes I don’t even know how it’ll be relevant, but I can’t be sure it won’t be.
Usually, in my case it’s a whole idea, e.g. a book about a woman who–ah, see almost gave it away. Instead, I wrote it down on my trusty APP to use later in some incarnation. Often I find that I’ll end up combining several different notes into one story or using a small piece of a larger idea in an unexpected way.
I also keep a folder of news clippings. A lot of the reading I do is non-fiction and much of what I write is inspired by real events, much more so than it is inspired by any of the fiction I read. I am inspired by that, too, of course. But the fiction I read tends to shape how I tell a story, not what that story is.
But the inspired-by-real events aspects of my books often turn up in unexpected ways. For instance, the author of the gRaCeFULLY blog in Reconstructing Amelia was inspired by a news story. I don’t want to give away that piece of the book’s mystery, but suffice it to say that the actual story I read bore no resemblance to what ended up on the page, but for me the two are thematically linked.
I’m also often inspired by other kinds of art–photography and installations in particular. If I’m ever feeling stymied with my own work, wandering around some of the amazing galleries in Chelsea always reminds me to dig deep and be brave.
But more than anything, what I write is inevitably inspired by emotions in my own life–fears usually (I am a mystery writer after all) about what could happen, or the consequences of what already has. The key, I think, is to find out what inspires you so that you can go there whenever you want. And to keep track of your ideas as they come along. That way your stories will be waiting for you whenever you need them.