When I was at Otakon I bought several "Junk" cels 
for $2 each so that I could try different 
cleaning methods for dirty cels.
I tried various types of alcohol- isopropyl, 
ethanol & methanol.
Isoproply will clean most grease and film from 
the cel. If left in contact with the paint it 
will begin to disolve.
Ethanol does the same as above but is harder to 
get.
Methanol will remove everything from paint to 
adhesive residue from scotch tape etc. Good for 
cleaning really bad paint smudged cels.Be careful 
when using this type of alcohol!

I have found that a clean soft cloth and Kodak 
Lens cleaner do a very nice job of removing 
finger prints and smudges. It does not attack the 
paint if you get some on it.

Care should be used when cleaning any cel.
Soft/lint free tissue or cloth should be used.
Use gentle presure when wiping, pay care to the 
lines that were painted/Xerox onto the cel.
They can be wiped right off!!

These are the things that I found. 
They work for me, but I won't promise they will 
work for you. Try them for yourself on a scrap 
cel first.

Regards-
Walt