An aside from my agent, sometime last year: "Just write another adult novel for me to sell." This came during a lull between drafts of Awake in the World, before I'd begun drafting the novel which is likely to follow that book. "An adult novel?" I recall protesting. "Teach me how to juggle multiple novels and a full-time career, and I'd be happy to." He didn't teach me, because that's a thing that you just have to learn, and I haven't yet.
But I'm getting there. Between late drafts of Awake, I wrote up a nice large proposal for the followup novel, and my editor and I worked through drafts of that as she kept editing Awake. Now I'm here: Awake is in the proofing phase, and I'm winding up the first draft of the next book, which I'll hand over to my editor in August.
That means, of course, there will be a small lull while she reads the book (and hangs lovely red marks all over it), during which I absolutely could, and should, be considering what I might do next.
That, I think, might be what my agent didn't tell me: that you write the other books during the gaps. There's never a pure, unadulterated stretch of time in which to write a book. Unless, of course, you haven't yet sold a book, in which case you have nothing but time. (If that's you: I envy you!) I've often wondered how popular authors manage a book or two or three per year, and it looks like this is probably it: they write in the gaps, always thinking about the next thing. (Nobody said the answer wasn't obvious.)
I don't know if I'll be good at that, naturally, but it can't hurt to think about being good at it.