You know how it is when you have all these stories in your head and not enough time to write them all down? So you decide which one is calling to you the loudest and concentrate on that one. Then, when that's done, you can check your list and decide on the next novel to write.
Once I finished The Kinshield Legacy, I started on the sequel, The Wayfarer King. I realized that TKL might not make it, in which case writing the sequel would be pointless (except for practice and my own satisfaction). Seeing as I'm writing for publication, I developed a strategy: set TWK aside until TKL sold, and work on something else in the mean time. So I started on two novels, one a fantasy and one a science fiction. I'd even started workshopping them through the private critique group to which I belong, The Prose Nest, and on the OWW (sff.onlinewritingworkshop.com).
Now that TKL has sold, do I pick up where I left off on TWK? Or do I come back to it later? For the answer, I turn to my publishing contract and read the option clause. My publisher has first right of publication on my next completed novel. Hmmm. It would make more sense to present the sequel to the book they're actually publishing, right? And if they decide not to publish it, I get back to the two others while I try to sell TWK elsewhere.