If you're going to do a temperature change, it's 
better to make it colder than making it warmer. I 
noticed that sketches would unstick much more 
easily after I just unboxed them and brought them 
into my house--- before they had a chance to 
acclimatize. It basically works on the principle 
that the different mediums (plastic/acetate, 
paint, and paper) will adjust to room temperature 
at different rates.

What you can do is subject it to a temperature 
change at a lower temperature, let it stabilize, 
and then bring it back to room temperature 
slowly. You'll find that this might be the 
easiest time to unstick... while it's still 
coming back to room temperature. You'll want to 
be very gentle and careful, so as not to inflict 
damage on the sketch, or suffer paint loss on the 
cel.

*Don't* put it in the freezer... it's too much of 
a sudden change, and is likely to warp the cel. I 
think the freezer might also have extra humidity 
which you don't want to deal with. (Water in any 
form is bad...) What I did for mine was put mine 
into a box (for safety), and put the box into my 
car's trunk, and left it there overnight. That 
way, it was able to gradually shift its 
temperature, and not suffer any shock.

"Gradual" and "gentle" are the key themes of this 
method. ;o) However, there are some cels which 
are just so stuck to their sketches that you need 
to accept their presence, or destroy the sketches 
during the removal process... and I'm such a 
sucker for sketches that I'd never think of it as 
an option. ;o)

Good luck,
-Cres