I came across this quite by accident the other day; in theory it's meant for novelists, but it applies to fic too
http://com1.runboard.com/bthescribesmes ... larea.t299
I think it's better if you take from it what you need; you may already have some systems in place for planning that you are quite happy with, and this may help you with others that you're not so great with.
It's helped me to realise that I DO need to do more planning than just a few notes scribbled into a text file (which I then never read again!). As a result the fic I'm working on at the moment has almost written itself already in terms of the plot. I just have to flesh it out with description and dialogue
What actually happened was that I tried to write the 'short description' of the fic, and the more of this 'short' (hah!) description I wrote, the more in-depth I got. It was almost refreshing to be able to write in all the elements I wanted to be there before I got bogged down in dialogue and description; I could change things later on in the fic and not have to re-write a whole chapter
It also meant that I could spot the 'holes' in my plot because the plot changes were physically so much closer together on the page, so I could remember exactly what had happened previously. The 'short' description has now turned into a complete 7-page plan! Almost by itself, I never seemed to get 'stuck' like I used to when writing pretty much off the top of my head.
Bah, I'm rambling again.
Hope someone finds this useful.