Well, here's the thing. Overwhelmingly, I'd recommend diversity. I have a trilogy (All the Windwracked Stars, The Sea thy Mistress, and By the Mountain Bound sitting in the trunk right now because I wrote all three books before I sold the first one. No guarantee that I'll ever be able to sell any of them.
Other than that, the answer is, you get lucky with an editor who really likes your work and is willing to take a mad chance on you.
What happened in my particular case is that I had queried arcaedia on the first novel of the trilogy and been requested to send her my synop and three. She read that, asked for the full MS, and then contacted me and said, "I'm sorry, but I can't sell this as a first novel. But do you have anything else finished?" (This process took over a year, by the way, so I had plenty of time to work on other projects.)
So I sent her the draft of HAMMERED and also fifty pages of another novel, a fantasy now called BLOOD & IRON, although this was three or four titles ago. And she contacted me back about three months later and said, "I'd like to represent you."
Huzzah! But wait, we're not out of the woods yet. *g*
I did some rewrites on HAMMERED to her request so that she could start looking for a publisher for it, and finished BLOOD & IRON and sent it off to her for comment. And then I started writing the sequel to HAMMERED, which is called SCARDOWN, unless that changes in press. arcaedia sent HAMMERED out to a publisher and we waited for a response.
Meanwhile (there's a lot of 'meanwhile' in this story), I finished two or three drafts of SCARDOWN and shipped that off to her, and started writing another book, called THE STRATFORD MAN, and a YA novel (THE COBBLER'S BOY) with a collaborator, truepenny.
arcaedia came back with some pretty stern comments on B&I (and she was right, as she usually is, but she sent me screaming into the night in a panic of "I can't do this! It's too hard!" for a while), and I back-burnered that while we finished THE COBBLER'S BOY and I kept working on THE STRATFORD MAN. truepenny's agent wasn't interested in representing non-speculative YA and arcaedia was, so we sent _that_ off to arcaedia once we had it finished. (Elapsed time now eighteen months or so.)
Anne Groell at Bantam Spectra called arcaedia with an offer on HAMMERED and "whatever I wrote next." This is where my smart, funny, good-looking, and very sharp agent really earns her measly 15% *g*
Since arcaedia had SCARDOWN sitting on her desk at that point, she told Anne that she'd seen a draft of the sequel, and that it was complete (HAMMERED completes an internal plot arc, but the external arc is left on a bit of a cliffhanger), and, since we had talked about my writing plan, arcaedia also mentioned that I had a three-book plot arc in mind. To make a long story short, Anne made an offer on all three, with an option on a fourth book. (Which will be BLOOD & IRON, or maybe the reworked ALL THE WINDWRACKED STARS.) And arcaedia handed me a bucketload of comments on SCARDOWN, and I finished the first draft of THE STRATFORD MAN and started rewriting SCARDOWN.
(Please note, at this point-- e Bear's score:
Novels completed: 8 (not counting some things that are trunked like an elephant going on a two-month vacation).
Novels sold: 3
Novels in circulation: 1
Novels waiting their turn in the barrel: *cough* 5, which would be 6, except I'm rewriting All The Windwracked Stars and The Sea Thy Mistress into one book.
Novels under contract but not yet written: 1
Novels begun, and waiting time to get them written, or a reasonable excuse to proceed--like selling whatever they're a sequel to--Whisky & Water, One-Eyed Jack, A Treachery of Princes, and two or three others of considerably less urgency even than those. I am unlikely to run out of ideas.
Complete ground-up rewrites (not draft revisions, but gutting the damned thing and more or less starting over) completed on spec, without being paid: 3
Fairly complex revisions of books that sold: 2
Elapsed time: 2 years.)
And that's why it's good to have fallback positions, and to keep your options open. Because the reasons I sold a not-really-a-trilogy aren't necessarily reproducible, and come down to right manuscript, right desk, right day.
Meanwhile, my collaborator, truepenny, sold two books (MELUSINE and KEKROPIA) to Ace about a month and a half before I sold HAMMERED, SCARDOWN, and WORLDWIRED to Bantam. But they decided to wait on picking up the second two books in what's planned as a four-book series.
She's certainly as good a writer as I am--perhaps better--and as fantasy, her work's likely to sell better (numbers on fantasy are usually better than numbers on science fiction). But I sold three books and she sold two. Freak show. Proud to be a part of it. Run while you can.
So really, just luck of the draw--although truepenny's books are starting in hardcover and mine are starting mass market paperback.
Also meanwhile, THE COBBLER'S BOY and its unwritten but planned sequels are still out looking for a home and collecting the flattering rejections--so selling one book quickly is no guarantee that the same fate will obtain for the next book, even if it's well-written, tightly plotted, and features a likeable protag. (Reading off our rejection slips here....)
So, I think the short form is that all you ever really sell is the book on the table, and whatever the editor in question is willing to gamble on your future. Unless you're a much higher-powered writer than any of us, you don't actually sell a series so much as get a lucky offer because an editor likes what he sees. So I would recommend polishing up that manuscript, looking for an agent, and working on something else, just in case that first one doesn't pan out.
Yours for transparency in publishing,