I have found several references to this, but as no-one knows what's on the internet tomorrow, I will post it here:
How to Plot When You Can't
(c) 1997 Sarah Smith
Here's a very simple, mechanical way to develop a plot. You can do this with index cards, if you're not a computer type. If you have a computer, it's a little easier. We'll do it the handmade way here and the computer-literate can look at "Software Solutions." [I don't know if the software suggested still exists or works.]
Start with some book ideas and a stack of index cards. Write down whatever you've got. For example, my initial ideas for a book always include an ending, some neat scenes, and some characters with goals.
Write each on a separate card, one thing per card. For example, "Miriam wants lots of money."
Then go through the cards and think about what cards ought to exist because these cards exist. For example, suppose your climax is going to be "Miriam throws Henry down the well." This card implies that Miriam exists, Henry exists, Miriam has some reason to get rid of Henry (the money?), and there's a well.
Write these down too, each on a separate index card.
Now go through those cards and think about what cards ought to exist because they do.
Cards can be questions: "Why does Miriam want money?" Or admonitions: "Make this interesting, not obvious!" Or scene bits: "Miriam rubs thousand-dollar bills on her skin."
Eventually you have a big stack of cards in no particular order.
Sort through the cards. Decide whether they belong to the beginning of the book, the middle, or the end. Most background and scene-setting happens in the first half, often the first third, of a book, so the card "There's a well" probably belongs in the beginning. So does "Miriam exists." "Miriam throws Henry down the well" probably belongs toward the end.
Some cards don't belong anywhere. They're Leftovers.
Now you have four piles of cards: Beginning, Middle, End, and Leftovers. Put the Leftovers aside for now.
Start with the cards in the "Beginning" pile. Sort them so they begin to tell a story. What's missing? Add that, one item per card. Adding cards may suggest other cards. Add them too, putting them in whatever pile and sequence seem appropriate. You'll end up with some ideas for scenes; you can rubber-band or paper-clip those cards together. You'll probably have a lot of material that doesn't seem to fit anywhere yet, and a lot of questions.
When you think you know what the beginning is about, go through the "Middle" pile. What can you add to the Middle pile that follows on from cards in the Beginning? Add it, one item per card, wherever seems appropriate. What characters disappear from the Middle? Should they disappear from the Beginning pile as well, or will you need them in the End?
Don't discard any cards; put them in the Leftovers.
In the End pile, you'll find more stuff that should have been prepared for in the Beginning and Middle, so you'll have to add cards to the Beginning and Middle or take cards out from the End.
When you're done, go through the Leftovers. Is there anything that you really want in the story, or anything necessary that you've left out? Where could you put it?
Look at the scenes again. Do the scenes flow naturally from each other? What cards--especially clues--could be inserted into scenes toward the beginning of the book, so you can have them available to dig up later?
Keep doing this until the piles of cards begin to look like a natural-born story and most of the cards are clipped together into scenes.
Then put all the index cards into a little box and begin to write the story. When you get stuck, use the index cards to remind you what scene you're supposed to be writing and what it's supposed to do.
That's all, folks.