Since I often have to sell cels when I buy new 
ones, I am pretty good at destroying attatchments 
to cels.  Here are two things I find helpful.

1) Find everything wrong you can with the cel.  
Nitpic every single line and inch of paint.  Put 
it next to better cels.  Find whatever you can 
that you dislike about the cel, be as picky as 
you need to be.

2) Think about how much the cel could mean to 
someone else, and how happy you may be making 
someone by making it available for them to own ^^