I've made up stories on-the-fly in my head for years...so fortunately when I actually sat down and wanted to properly write a novel, I knew that if I just winged it, I would end up with a huge convoluted mess of coincidences, plot holes, and everybody being related to each other. Though my first novel ended up that way anyways.

These days I plan to some extent - I don't outline, but I keep a general plan of what will happen in my head, and might occasionally write down notes. As I do chores and school and so on, I imagine scenes from my novel and think about the characters and plot, and then often I run into interesting details or twists I could add. So I just leave it percolating for a while, you could say.
I've never planned out an ending, though. With my current novel, for instance, I know how my MC's character development ends...but I don't know how the actual events end! I figure that once I've written all the way to an ending, I'll have a good sense of what should be in it...so I just foist the ending off on my future self, really.

I have completely pantsed short stories, though. Two times, I suddenly thought of an interesting opening line, and then I just sat down and wrote from there. Both of them turned into allegories full of Symbolism and Meaning...the kind of thing that always happens when I write on the fly...
