My eagerly awaited Ryoko cel came today.
I bought the cel knowing it wasn't in perfect shape,
but I felt hopeful and adventurous and decided to try
my hand at restoring her to at least some of her former
glory.
I had some disappointment with rubbing alcohol- I had heard
you could clean the non-paint side of a cel with it, and
to be fair, I scrubbed a bit at a line with it (end of one 
of the hair tips- one of the ones still in perfect shape)
and it didn't seem to fade the line, but still the lines
looked a lot choppier later- there was a lot of dirt that
came off, but it looks like some of it was residing in the
lines. So maybe cleanliness isn't always so good.
But anyway, the story goes on.  I used my fingernails to
scrape off whatever was stuck to the top of the cel with
fairly good results, and the alcohol may have helped with
that clean up.  I couldn't get a little in her hair and may
fight a bit more with the spot to her left- but here's what
I've got - left-before, right- after.