Fading has to be THE worst thing on a cel for 
me.  Id rather have a chipped cel than a faded 
cel ;\.  I've noticed that one of my cels is 
starting to exhibit some fading too.  It's minor 
at this point, but it's my favorite cel.  Im also 
trying to figure out what to do with it.  The 
first cel I ever bought I had in a completely 
sealed bag, in a large shoe box in my closet.  
That's where it stayed for about 3 years.  I 
didnt even look at it...I forgot I had it!  But, 
the lines are still absolutely perfect on this 
cel!  Then again, I know a girl who keeps some of 
her cels in totally open bags and her cels are in 
good condition too.  I think it just depends on 
how well the cels were cared for in the 
beginning, the climate you live in, and the 
materials used on the cel.  I dont know the 
answer either...all I can say is: dark, constant 
mild temp, no moisture.  That shoe box might 
actually be where I put all my cels!  It seemed 
to be awesoem!