One of the lessons I've learned in my past few months working with an editor is this: if you have the character backgrounds pretty firmly fixed in your mind, you might save yourself some rewriting.
I'm getting ready to start a new book while I wait for the verdict on TSMR. I've done several things that aren't like me. I've written a synopsis — I never do this, except when forced, and I know things will probably change as I go along anyway (it's a good thing to be forced, actually, because it makes me think through issues I won't otherwise). I've written the character backgrounds — this helps me know why they do some of the things they do. Who are they? Where do they come from? What are their issues? What do they want now?
These are important things to know. If I throw two characters into a situation without knowing who they really are and what made them that way, then how can I have a coherent story? Because they could do anything for the sake of plot if I don't know them. It's when you know what your characters would and wouldn't do that you are ready to write their story.
I still consider this pantsing, btw. Plotting, to me, is mapping out everything from the first kiss to the first turning point, the black moment, the resolution, etc. And then not deviating from it, or deviating only slightly. I need more room than that because I grow bored otherwise.
But not so much room I rewrite the book three times. I'm determined to slice those rewrites in half at the minimum. So I'm writing backgrounds, forming a synopsis, and brainstorming 20 things that *could* happen during the course of the story (this is a trick I gleaned from a Jane Porter workshop). I already know the first scene, and I'm beginning that today. I may learn something new as I go along, and I may have to rewrite a scene or two. But I really, really hope I'm figuring out how to cut the rewrites down to size with all this pre-writing.
If you're a panster, do you do any prewriting? If you're a plotter, how much plotting do you really do before you begin? And do you ever deviate from what you've predetermined? How do you plot and not feel stifled by the roadmap you're created?