I sort of use a combination approach to plotting, Mark.
There is a certain amount of planning and outlining done in advance, if only so that I can be sure there really is a story there (as opposed to an idea for a story -- which I've learned is not the same thing at all) and that the story has someplace to go. But the planning gets less and less detailed toward the end of the outline, and I tend to regard what there is of it as sort of a default, because I want to stay as open as possible to changes wherever a problem appears, or whenever something good comes along unexpectedly.
Also, with a really complicated plot too much pre-planning can be a big mistake, I think. After all, the beginning of a project, before I even start writing, is not the time when I know the characters and their situation and the setting as well as I'm going to. It's not a time when I've had a chance to figure out all of the complications and ramifications that could possibly come up. So I try not to marry myself to any half-baked ideas just because they looked good at the beginning. (Note that I say try. I'd be the last to claim that I always succeed.) And the more characters and plotlines going on, the easier it is to avoid gaping plotholes if I let the story grow naturally, one thing leading to another, ideas generating other ideas, instead of start out with a bunch of totally unrelated (but cool) elements and trying to graft then all together after the fact.
But very little of the above applies to a more linear plot. With short stories, for instance, I pretty much have everything planned out before I start writing.
Another thing I do with a long project is think a lot about what's coming up next and what's coming up later, and end up scribbling down a lot of bits and pieces -- sometimes just my thoughts, sometimes actual pieces of dialogue or narrative or description. I used to put all of this into spiral notebooks, which tended to be pretty much a mish-mash, since none of these things come out in any particular order, but now I'm learning to write things down on note cards or sheets of paper, so I can pop each one into a folder devoted to the appropriate segment of the story. Would that I had done this from the beginning, because I still spend a lot of time leafing through notebooks trying to find things that I know are in there somewhere ...
But it often happens that by the time I get to the chapter or the scene in question, some of these notes no longer apply, and I end up not using them after all.
As I said, a combination approach.
