Infinity Archival See-Thru Mounting Strips by 
Lineco (Item No. L533-4015) which allow you to 
mount cels to a mat or a backing board with no 
adhesive touching the cel.  I would be concerned 
about the safety of any adhesive in contact with 
a cel over the long term.  The strips are about 
$16 for a box of 60 at local art stores.  

They look like the figure below, a strip of mylar 
glued to a strip of archival mat material, 
between which you wedge the cel.  The mat strip 
serves to raise the cel above the backing board 
and has an adhesive surface that lets you stick 
it to the backing (or to a framing mat if you've 
got a production or laser background).  A cross-
section looks something like this:

mylar -------------------  
mat   xxxxxxxxxx          

They're about 4 inches long, with the mylar strip 
being about 1 inch wide and the mat strip about 
3/4 inch wide.