@MushroomI never outline, formally, that is. Rather, I typically plan out the whole story in my head, and then sit on it for months, waiting until everything gels, often throwing a monkey wrench into the story to complicate things, and letting my subconscious work out the details (i.e. how to resolve the storytelling issues), so that when I finally start, I've got a complex tale with plenty of juicy subplots.
Recently though, I've largely been going without, getting a general idea and just writing, but that really hasn't worked out in the least! But as I'm getting older, that's an awful lot of work and a serious time investment, just as my gray matter is being stretched a little thin.
The advantage though, is that if I don't start writing it immediately, the plot goes stale as my assumptions grow too rigid to evolve naturally as I tell the story, meaning anything that takes too long jump gets dumped outright. In my later stories, there's not that defensive wall, and as long as it passes my 'three-chapter' viability test, I just plow ahead, writing it chapter by chapter, and the complexity eventually falls by the wayside.
But, even when I do plan it out in advance, my mental outlines are vague, as I work out the beginning, ending and the major waypoints, but never the exact details. So, when I say I know how the story will end, the details are vague. Say, if the protagonist ends up dying (a common occurrence, as I've always been a fan of the 'sacrificial hero' motif), I'll know that he dies, but now how, so I'm still surprised when it actually happens, and share the same shock I'm hoping to convey to the readers.
In short, too much planning essentially kills a story's spontaneity, but too little can be deadly too, as you end up venturing down dead-end paths without actually 'thinking them out'. At least for me, creativity is best left to the subsconsious, as the logical, detail oriented left brain runs rampant of the more creative right side of the brain. If you write out the full story before editing, like many of us do here, there's plenty of time to flesh out the details and make any necessary corrections, including adding foreshadowing and enough red herrings to leave the conclusion in doubt (so the ending makes sense, when it occurs, though certainly not expected).
So, I mentally outline using my subconscious, write straight through without editing, then once I know how everything works out in the end, head back and do some extensive revisions, knowing where the story is headed I can then tailor the story to lead the readers there. However, writing each chapter on the fly you lose virtually ALL of those advantages! But as I already stated, that approach takes a LOT of work!
The general idea though, is to just outline the direction you want to take, and then leave the pantsing for each chapter.