There is Plexiglass cleaner you can buy, which 
looks kinda milky.  Glass cleaner should not be 
used because it often contains ammonia which 
melts plexiglass and thus clouds the plexi over 
time.
Anyhow, I would use the plexicleaner.  Also, 
there are clear archival things you can buy to 
but between the cel and the plexiglass.  You can 
use mylar, which is good, but if you want 
something truly archival, there is something 
called dartex.  It might be kind of expensive and 
I'm not sure you can only buy a small amount vs. 
a huge roll of it.  That is what I would use on 
an expensive cel though, so it's not touching 
anything else which can have a negative effect on 
the cel over time.

Hope that helped (I work in a museum  :)
Mark...

PS:  to use the plexi cleaner, spray the cleaner 
generously on a rag to wipe the surface, and then 
follow immediately with a clean dry rag to buff 
it dry.  This gives you a very clean finish.  
Soft cotton rags = good.