I'm one of those unstructured writers that everyone in creative writing class laughs at.
Instead of developing the story as I'm writing the first draft, however, i've started writing a treatment. It's similar to an outline, but not as rigid and formulaic (sp?). It's used in screenwriting to pitch the full lenght script. It's like a 5, 10, 15 or whatever page version, written in complete prose, covering the core plot and character arcs. No details or art about the treatment, just: "John and Susan use their newfound knowledge to break into Mr. Burke's office. After some rummaging, they discover bomb blueprints and detailed city maps in a hidden compartment in the wall. As they discuss the revelation that their boss is a villainous mastermind, they hear footsteps in the hallway and Mr. Burke's familiar whistle." And so on and so forth, through all the major scenes of the book.
This method lets me avoid the rigidity of an outline, yet lets me develop the plot and characters before I begin the actual first draft. And after I do the treatment I can transfer it all to notecards if I wish, but I don't personally. Notecards are too structured for me.
Those are my two cents.
BLG