Wait for it
All of the chaptered fics I have posted so far I got the rough draft of the whole story done and then I would edit each chapter before posting them. I guess part of the reason is I tend to change things around a lot with my stories even after I get a few chapters written already and I can easily see which parts need total rework or just a bit of tweeking. To be honest I don't think I can imagine myself posting a chaptered fic without the whole fic finished. @_@
As for planning, I tend to think the beginning and end while the middle I leave it wide open and come up what will happen when I get to that point. Sometimes I would changes plans if I thought up better ideas over the execution of the scenes.