Hmm... Well, I think I'll take this one on.  (What, me, respond publically
to something?)

I think it depends on both the author and the effect the author is going
for.  Take, for example, the television show 'Babylon 5.'  VERY well
planned show.  The creator, J. Michael Strazinsky (sp?), came up with the
history of his universe for 1000 years before and 1000 years after his
show.  Then, he refined it, making a more specific history for 100 years,
both ways.  Then, even more specific for 20 years.  Then more specific for
5 years.  THEN he began planning the first show...  I think most people
will agree, the results are outstanding.

On the other hand, many famous authors- like Jack Kerouac- took out a sheet
of paper (or a roll of paper, as the case may be), stuck it in there
typewriter with no idea of what they're trying to write, and just start
writing.   They don't stop until they run out of paper, they don't go back
to fix anything... it's all very 'stream of consciousness.'

Still others (though no specific examples come to mind) like to come up
with a very basic plot, like, say, (using a really bad example), 'Ranma and
Ryouga kill each other in a fight.'  Then they might come up with a basic
outline, like...
I.  Ryouga comes up to Ranma and challenges him to a duel
II.  Ranma gains the upper hand, but is lead by Ryouga to a high cliff face
III.  Ryouga does a Bakusai Tenketsu, destroying the cliff face and sending
Ranma and Ryouga both plummetting to their doom
IV.  Akane mourns the passing of both Ranma and Ryouga, then goes and runs
off with Shinnosuke

Then they'd take that VERY BASIC outline, and run with it.  They might come
up with a specific reason why Ryouga comes to challenge Ranma, they might
decide that Ryouga INTENTIONALLY led Ranma to the cliff (or might not). 
Maybe, before plummeting to their doom, they nearly save each other, and
finally come to a reconcilliation seconds before the rock crumbles and they
plummet to their doom.  And then, maybe Akane goes to Shinnosuke because
she's trying to remember some of her favorite times with Ranma, and finds
Shinnosuke again walking the path where she and Ranma held hands.

And so on... just build it up from the outline.

And, there are probably ways to plan fanfics which I'm not familiar with,
and there are probably some which are a combination of the above. 
Nevertheless, I'd say there is no ONE way to plan a good fic, and that it
is not for others to decide...
----------------------------------------------------------------------------
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Sir Desaix, member # 116 of the Knights of the True Fiancee
               (Arrrgh!  They Mispelled my Name!)
                             desaix@sysnet.net
    http://www.geocities.com/Athens/Acropolis/7872
                           fanfics available at 
http://www.geocities.com/Athens/Acropolis/7872/fanfics.htm