How do you plan your writing?

I haven't posted for a while as I've been recklessly busy. One day I'm going to turn up at the wrong bookstore on the wrong day! In the ten days since my last post, I've signed a lot of books - at Waterstones in Hitchin, Waterstones in Harrow, and WH Smith's in Brent Cross. Tomorrow I'm on a panel of Crime Writers at Gayton Library in Harrow, then off to sign at Waterstones in Norwich on Saturday.
ROAD CLOSED officially launched this week.
DEAD END, which will be published in 2011, is with my small circle of readers who are beginning to feed back comments. There's still a little more work to do on the manuscript - and I really need to sit down for a couple of days and read through it closely but also fast so I have every detail in my head. I'll tackle that task after I've broken up from school next week as I want to give it its due.
I've decided on the title for my fourth book in the series - but you'll have to wait and see what that is. This week I've been working out the plot and am getting very very excited about it. I can't wait to finish the synopsis my agent wisely insists on so I can start writing it. In the meantime, I haven't even finished my plan yet - but my research is under way and my plan is moving forward.
Having completed three books, I feel like quite an old hand now! So this is how I work.
1) make a sketchy plan on A3 paper. I divide the sheet into five columns, one for each part of my book, and I list the chapters down the columns, a chapter name and very brief notes to remind me what the chapter is about.
2) write a chapter by chapter synopsis from my plan, giving details
3) write the book
That should be it, but of course there's more
4) realise something doesn't work - rethink and rewrite
5) have a brainwave - rethink and rewrite
6) notice a dreadful gap (or gaffe) in the plot and have to revise completely...
That's just the initial stages of writing a book. Then come the agent, publisher and editor... and more rethinking and rewriting...
So forgive me if I don't post for a while. You know what I'll be doing, when I'm not at school.
So far I'm at number 1) and have already radically changed the whole idea twice. Now I've come up with an idea that I'm happy with, I need to complete my A3 plan... How will Geraldine solve the mystery? Is there enough suspense in the plan? Who, exactly, is my killer - there are so many possible characters I could create to play the villain here, but who shall I choose? And what is in store for Geraldine in book 4?
I wonder how other people plan their books?