Try using a peg board, they fit the registration 
holes from the cels and hence keep them in 
position (usually used to keep the cel and sketch 
aligned etc). You can get them from animation art 
suppliers like Chromacolour for a couple of 
bucks. You just need to then fix it in a position 
on the scanner (try tape etc). Though as others 
suggest you may have scanning issues in keeping 
the colours the same etc.

Cheers

Michael