Well, mostly I think about an idea constantly non-stop until I have enough of
a semblance to an outline to jot it down.  When taking shower's, going to
sleep, dentists office, lecture ^_^, essentially occasions where my mind has
nothing better to do other than wander.  They'd beat around in my head often
enough where I'd jot it down on my computer.  Ranma/SM was contrived without
my knowledge that there were a million crossover's out there already.  I had
discovered Ranma not too long ago, and my anime knowledge was limited to
those two at the time.  Images of Ranma fighting SM characters was the
genesis.  And in all honesty, the story was never going to be more than that.
 But after I set up the Happosai scenerio where they'd meet, the negaverse
just came naturally and then WHAMMO!  I had a full fledged plot I couldn't
help but write.  ANd I guess Ranma: Prom Night was similar to that.  I
basically write around "events."  I think of an event, Ranma vs. SM, a prom,
a must sequel to Ranma/SM.  I think of HOW to get there, and what springs off
from it.  
I'd just like to add that I completely agree with Ashira regarding writing.
 As the initial thrill of fanfic writing has worn down, you really to have to
push yourself to not be lazy and jot down all those idea's you have in your
head.  People who read my fanfics now that I slowed down from 10 pgs. a day
to 10 pgs. a month.  And that eventually lead to my retirement after one more
fanfic...  Or maybe I'll actually get that Ranma/SM sequel out after Four
Fiancee's...  And then there's that Fushigi Yuugi/Ranma thing I hinted at in
PN...  And also that idea about...  ^_^  But in all honesty, towards the tail
end of PN, I wasn't writing for "fun" as much as I was writing out of an
obligation to my readers.  And I think from now on, I'm writing more to make
my reader's happy than to make myself happy...  Not that I don't have a good
time writing, and the adulation from a few people doesn't hurt either...
Jya Ne
Shiaw Chi "Hokodan"