Well, stuck layers on multilayered cels is almost 
unnavoidable.  Unless the people who get the cel 
when it's released take the care to put plastic 
between the layers, they will get stuck :(  I 
have a few multilayered cels that are stuck like 
that.  It really doesnt hurt them at all.  Now, 
being stuck to the sketch is another issue.  I 
HATE that to death.  Two very nice cels I have 
are stuck to their drawings.  Some people say 
putting your cels in the freezer for a bit of 
time helps, but there's no way Im putting a 500$ 
cel in my fridge on a chance it'd get harmed.  I 
dont know a safe way to unstick them if they're 
really bad, but I do know a place that can 
unstick them safely for a reasonable price.  It 
takes about two weeks to get the cel back, but 
it's worth it.  As soon as my cash flow is 
constant Im sending my two in ^_^. 

http://www.srlabs.com/