Here's my reply to some comments on outlining stories in a discussion on the John Connolly message boards. Reusing it here is a lot easier than coming up with something original. Lazy, me?
>>Every now and again I get images and flashes of scenes, which I work toward. But they hardly ever end up in the finished story.
I think a lot of people tend to see outlining vs. making it up as you go as much more separate concepts than they really are. If you're outlining you don't have to stick to your outline, if you get a better idea once you start writing then use it. Similarly if you're making it up as you go and an idea suddenly pops into your head the idea isn't necessarily going to be something you can use at that exact moment in the story, you have to keep hold of the idea until you can fit it in, usually by steering the story towards scenes that will support that idea. Even if the idea is closely related to the scene you're writing -- e.g. "Hey, I just thought of a great way to end this scene!" -- you've still got to write the rest of the scene first and that means you're planning, if only only a very small scale.
Personally even when I'm working from a very detailed outline it tends to deal more with story structure than the actual words I'm going to use. So there's still a lot of improvisation in terms of the dialogue and descriptive writing.
Michael Moorcock discusses this at length in Death is No Obstacle. He is a big fan of outlining a story's structure but he still makes things up as he goes. He likens it to jazz improvisation, if he finds that he has used a few similies or metaphors relating to say, mirrors he doesn't think, "Oh crap, I'd better change that so I'm not constantly repeating myself" instead he turns it into a motif and tries to tie it in with the themes of the story.
Sorry, I'm not trying to come across a know-it-all, I realise that the writing process is different for everyone. A lot of times when I said "you" in this post I probably should have said "I" but I was just trying to give my ramblings a little more sense of immediacy.
Basically there's no one surefire way to write a story. Just use whatever works for you. But if your usual method suddenly stops working for whatever reason (it's happened to me) remember that there's other methods out there you can experiment with until normal service is resumed.