I was a little upset the other day when I took an 
older Devil Hunter Yohko cel out of the bag to 
scan it (I just recieved the cel but the episode 
probably came out more than 10 years ago) and 
found a large peice of paint had simply seperated 
from the acetate.  It didn't seem damaged- the 
color and texture seemed fine- it just didn't 
adhere to the cel anymore. 
Since it was a cheap cel I sort of "glued" it 
back on by putting the spot in its proper place 
and painting a relatively thick coat of matching 
acrylic paint over the top and to the surrounding 
areas. Once it dried it seemed to hold, but I can 
see that of course the original paint is still 
not adhered directly to the acetate.
ON TO THE POINT....
How to prevent such a thing? It's one thing if a 
bit of paint falls off the bottom of the clothing 
of a character in a $25 cel, but when we're 
talking cels that cost 10 times that or more, 
that's a very serious thing. I keep almost all of 
my cels in bags, and they are stored flat, out of 
sunlight, within the plastic sleeves of a cel 
book.  Is there anything more we can do to save 
our cels?