I was lurking over at the Absolute Write forums, as I do occasionaly, and there was a thread about working on multiple projects at one time.
The general wisdom of this (although AW had a number of people bucking the trend) was that you shouldn't get distracted by your Shiny New Idea, because this way lies madness and the Not Finishing Of All Things. You start a project and you plow through it and if you get an amazing idea while in the doldrums of the middle, you jot it down in a notebook and then you go write back and you clean your plate, missy, there are authors starving in garrets somewhere who would be grateful for that wordcount.
So I started writing up a comment and it ran long enough that I figured it was probably a blog post in the making, so...um...endure my rambling commentary, internet! (Or don't. It's cool.)
I do the thing that common wisdom says not to do. I do it a lot, in fact.
I work on multiple projects at a time, because if I didn't, my publisher would be sad and then I would be much sadder! I think most productive writers who are doing this full time are at least always, at bare minimum, editing one and writing another, just to make the whole system work.
But also I do it because that's just the way I work, and I do it very hard and kind of to extremes.
My general method is to start something, work for awhile, hit a sort of natural stopping point (usually the bit where I would send it to my agent, actually) and then go work on another project. Then it might go on hold until A) it sells B) I get an idea where it goes next C) I wonder vaguely what happened to that character D) it occurs to me that I really need to start working on something with an eye to self-publishing it at some point in the future, and hey, I already started that one thing, that's ten thousand words I don't have to write from scratch, let's see where we stand...
If I get a shiny new idea, I absolutely chase it. I chase shiny things like you wouldn't believe. If I am suddenly passionate about X or Y or Z, it would be stupid to waste it. I've got plodding determined work ethic coming out my ears, but mad passion is in very limited supply. Any day that I can whip out three thousand words in two hours is a damn fine day. (I wrote the first Hamster Princess in two days. It is about fifteen thousand words. I cackled while I wrote it. It sold within a week. Castle Hangnail took like two weeks for the first ten thousand words, and they bought it so fast that the contract practically hit me in the back of the head. Chasing Shiny New Ideas has worked really really well for me.)
And having chased down three or five thousand words (or ten, or fifteen--rarely more at once) I toss it in my hard drive, and then when I get that itch to work on it more, I pull it up and throw another five or ten thousand words on it, until I drift away to something else.
Currently in the "just started this but clearly live" phase: A weirdass version of the Goose Girl with an evil horse, the story about the girl who built her dog out of bones, and a version of The Firebird staring Grandma Harken and an enchanted mockingbird stealing her tomatoes.
Currently in the "well along and adding words now and again" phase: The one with the girl with social anxiety and the dog named Copper, a retelling of Tatterhood, the second Goblin book, the Regency version of East of the Sun, West of the Moon with Master Rat, the girl who gets sent to another world by Baba Yaga, the one with the ninja accountant, the retelling of Sleeping Beauty with the changeling raised by greenteeth and the Muslim knight, and the night-gaunt romance (although that's rapidly being cannibalized for spare parts for this middle grade novel.)
And there's like three that are on hold indefinitely until I figure out what to do with them, which include The One With The Armadillo And The Kinda Unlikable Kid, The One With The Barbarian Gynecologist, and The One With The Moth-Riders. They may be dead, they may not be. They are at least waiting for something to click that hasn't yet. (Then again, the ninja accountant one was also on hold until last year, when I suddenly threw another ten thousand words at it, so y'know.)
I do not actually have many trunk novels lying around. Partly I am arrogant enough to think that I can probably pull SOMETHING out of any given project, and partly all my early writing was on an Amiga and probably now on a zip disk somewhere in my ex-husband's attic. There were two reasonably bad novels on that, but one of them I still cannibalize occasionally.* Otherwise, I don't know of anything I'd call Officially Trunked Forever.
(Special mention of Regency Ninja here, which is on hold until my agent sells it, which she is determined to do come hell or high water.)
(Probably I am forgetting at least one project. Someday I will turn it up on my hard drive and go "Oh, WOW! Holy crap! Let me finish this!")
(I like parentheses.)
(I talk in parentheticals sometimes. My friends are very patient.)
Assuming that a story does not fall into the indefinite hold, there comes a point--usually around the 30-40K mark, interestingly enough--where it snowballs and I grab it and crash through the end.
This is a terrible method by all objective measures and all I can offer in my defense is that it works great for me.
At the moment, I've got one for self-pub in the snowballing-to-the-end mode, (the Snow Queen one) and I've also got a middle-grade novel that is about 11K along, except that it has already sold, so I don't have the luxury of my preferred work method and have to work on it straight through. (Which I am capable of doing, particularly if someone waves money in my direction. See above about plodding determined work ethic..)
So the last two weeks, I literally go to the coffee shop, open both files, and write 500 words on one, then 500 words on the other. They are wildly different. If I want them both done soon, this is my only real option. (And in a month or two, I'm going to have to swap out the Snow Queen one, finished or not, and hammer out the rest of Hamster Princess 4.)
(The next one in the chamber after the Snow Queen is probably the retelling of Tatterhood.)
If I absolutely catch fire on one, I will drop 1300 words on it and hold off on the other one. It happens about once a week. But most days--four days a week, one coffee with cream, one refill--I go and put 500 minimum on one, 500 minimum on the other.
There is absolutely no question that this method won't work for everyone. It might not work for the vast majority! I have a hard drive littered with false starts, or at least starts that haven't gone anywhere yet and may never. (The rusalka and the twins! The widow and the horse of power! Maggie Gray from Pocosin and the enchanted fish! The girl who is reflected the right way around in mirrors and the one weird thing with the followers of an evil version of Janus walking through doorways at night to try to wear a hole in the world.) If I never come back to any of those, I may never feel the lack. Or I may start working on one on a whim and look up and find that it's another five thousand words along and starting to feel like a live project. Stuff happens.
This is also a tricky method in that it may take up to eight years to get a story from initial Shiny Idea to published novel. I started Seventh Bride in 2006. But there's also always a bunch of other work in the pipe ready to be completed, so if I finish a book tomorrow, I don't have to go back to the drawing board. I've got plenty of things half-completed lying around already, so I can still put out work pretty reliably.
My friend Mur asks how I keep this stuff in my head. I don't know. Badly, probably. I forget stuff all the time, and then I get excited when I re-read it, because dude, that was awesome! I gotta write more! Still, it works out. In not quite ten years, I've written 17 middle-grade books**, two adult novels, two novellas and a pack of short stories, and I'm not even including Digger in there (though that was a one-foot-after-another march, not my usual flitting from project to project.)
Lord willing and the creek don't rise, in the next ten years I'll finish off a bunch more.
Look, do the thing that works for you. If you are a one-book at a time writer, go for it. Do that. If that means you finish the book, then finish it. Finish it like the wind!
But don't let anybody tell you that spending a week hammering out the basis of Shiny New Idea, and flitting between projects like a butterfly with itchy feet, is absolutely a bad thing that will lead to artistic ruin, or whatever. I do it all the time. As long as you know you go back to the old projects and finish them (and you know you, I don't--if you say that you finish stuff and will go back, I will believe you) then hey, the system works.
And that is all the writing advice I have today.
*I will by god! get a blood-drinking hummingbird familiar in something if it by god! kills me! Also the mirror assassins made of glass and the thing with the white deer woman and the visions. I already recycled the crow with two pupils in each eye in Cryptic Stitching.
**Bread Wizard counts as written, damnit, even if the limbo it was in got very, very weird. Publishing is surreal. But it has been mostly freed of said limbo and may even be published someday.