I've tried alcohol to get them off -- just be 
VERY CAREFUL with it. The stuff can remove paint 
and trace lines, if you use it to remove 
fingerprints on the back of the cel. I usually 
use a Q-tip and blot/wipe it off almost 
immediately afterwards. Alcohol's also good for 
getting rid of excess paint on the cel, and a 
generalized cleaner. 

Found in the archives a method to clean the 
cloudiness from cels. Check under mineral oil in 
message. Hachi-san (sorry if I'm spelling your 
name wrong), says that mineral oil is good for 
getting rid of it.