First, you need some non-abrasive cloths (you can 
try asking for polishing/cleaning cloths at a 
photo/art store).  You'll need a cloth for 
mineral oil, and one for the soapy water.  You'll 
also need cloths to wipe off the oil and water.

Mineral oil (easily found in the medicine section 
of a supermarket) can remove the whitish 
spots/specks from cels.  In case you accidently 
get it on the paint, you can easily wipe it off 
without damaging the cel.

However, mineral oil will leave a residue 
behind.  Soapy water will fix that (and remove 
other residues).  Again, if you accidently get 
soapy water on the paint, you can wipe it off 
before any damage occurs.

A word of warning for airbrushed cels, even a dry 
cloth can wipe off airbrush effects, so stay 
clear away.  Later.