One of the things that I do, or at least that I try to do, is to write an idea down as soon as I have it, even if I’m in the middle of another project. Because 1) that’s when all my best ideas occur (it’s the grass is always greener symptom of writing), and 2) most ideas need some time to simmer in your brain anyways. At least, they do for me.
I have a whole folder entitled ‘Other Ideas’ on my computer, that contains every half-baked and random snippet that pops into my head. You never know what random idea you had at 2:51 PM on a Monday is going to turn into a full-fledged novel. Or when a perfect line drops into your head, but you don’t have a book for it. The important thing is to write it down, because who knows what you’ll think of it in a couple of days, weeks, months, or even years.
Another reason this is helpful is because it gives you options. I never understand people who don’t have projects lying on the back burner, only because I have half a dozen at any given time. Now, obviously not all of those are going to be viable novels, but it’s a place to start. But any idea that has even a little bit of stretch to it goes in the file. And eventually, at some point, I’ll open that file back up and play around with it a little.
The important thing is to record those moments of inspiration, because you never know where they’ll lead. Moonset started from a single line of dialogue that didn’t even make it into the final book, but summarized it completely. I didn’t work on it for almost a year after writing down that first line, but as soon as I did the story came rushing out of me.