I tend to wing it fairly heavily, very seldom writing to an outline. (Never do this.)
Description is easier for me than plot. I just noticed how my habit of trying to visualize and define setting helps me build the story like a Tetris game, using whatever blocks "fall down" first, seemingly by chance. It happens even when the blocks "disappear", so to speak. I tend to write too much description, and then hack out some of it to make sentences flow better. But it's in my head, and the dropped parts may reappear in a functional capacity, later.
1) I needed Lulu to get kidnapped by somebody or other. While describing the attack, I suddenly found myself describing Sin boiling under the water. Pretty quickly, I had a much more detailed plot than before -- it's not just a random gang of folks making a power-grab by seizing friends of High Summoner Yuna; they're trying to use Sin as a cover for their operation. Which provides an interesting secondary reason for Lulu's capture.
2) I was trying to describe a motley ragtag crew of several different races. I idly mentioned "even a Guado". Before I knew it, he was the captain, because ships need a captain, eh? Then, of course, the captain would talk to his captive at some point. In my efforts to describe a Guado's mannerisms, keeping ol' Trommel's voice somewhat in mind, he quickly developed speech patterns, gestures, clothes, and at least some personality, though it's pretty standard Smarmy Villain material.
3) I needed a destination. I called up a mental image of the world map. I named the first out-of-the-way spot that came into my head: Baaj. And it is at least off the same coast of the continent as Luca, where the ferry would've been coming or going. Then I remembered that Seymour and his Mum holed up there. Whee, Guado connection! I set to work describing the place.
4) I needed to get Y,R,T close to Lulu somehow. Infiltrate mansion? That was getting hard to visualize. Swim? There's a thought. In the course of describing/visualizing the area outside of Baaj temple, I came up with the idea of hopping aboard the U-boat from one of the ruined towers. Woot, rescue plan.
5) In describing the dock, I idly mentioned some flatbed loading carts and crates, just because I was trying to fill out a basic picture of the setting. One of them became a handy prop in the chapter I just wrote, solving a problem I was having with a fight sequence.
So anyway. Weird how often some random detail you throw in early on can serve as the foundation on which important bits of the story are built up, like Tetris or a snowflake.