An incredibly well-timed post from io9: Great Science Fiction and Fantasy Writers Who Never Wrote Sequels or Trilogies.
"Well-timed" in big part because I was just debating this very issue with others earlier today, and because it's something I've taken a stance on re: my own work. No sequels, no multiple works in the same universe.
That said, I am fully prepared to admit I might reconsider once I have to deal with the way publishing works apart from hustling individual copies across a table at a convention.
Everything I've written up to date, I've approached with the mind-set that I'm best off whittling it to fit into one comfortably-sized work. Tell the story, tell as much (or as little) of it as is needed, get it done, move on to the next thing.
There's a couple of reasons I've taken this approach. One is because I've come to feel the world you pick for your story is crucial, and if you start from scratch each time you give yourself that much more leeway and latitude to shape both the story and the world as you see fit. I have tentative plans for a story after Flight of the Vajra which would need an entirely different milieu to tell it properly. There really isn't a way to tell it in the same universe as that story — well, there might be, but at such a cost to the story that I want to tell that it wouldn't be worth it.
The second big reason is time. I don't like the idea of repeating myself, and writing more than one work set in the same universe just feels to me like retreading. Yes, even if different characters are involved, for the reasons I cited above. I'd rather break away and do something entirely different. I have no idea how long I'm going to live (not saying I have something terminal, just that it's an unknown), and I know that when I feel like I'm repeating myself I only regret the feeling. I don't want to waste what I've been given.
Also note that I'm not trying to use this as a critical mode for other people's work. I'm fondest of when someone is able to tell the whole story they have in mind in a single, well-considered volume — and that's more where my tastes as a reader lean these days — but I'm scarcely going to tut-tut you if you're reading Book Five of the Whatever Cycle.
So if I can't say anything else about Vajra yet — and I have some carefully-selected details in the works — I'll say this: it will most likely be a single, standalone story. It is not part of a series and will have no sequels, prequels, spinoffs or side stories — and that's the way I want it.
I should add that I'm also prepared to come back to these words in five years and say "God, was I wrong about that."
By the way, the authors of that piece get major props for mentioning Alfred Bester, Theodore Sturgeon, Clifford Simak and Phil Dick — all of whom occupy places of honor on the shelf in here.