Cels Forum

If you really don't care about the pencil that much(which
to me seems obvious since you're willing to soak it with
a sponge) then you could just try carefully tearing it away.
Keep your cel flat, then put your finger along the edge of where it's stuck and pull gently.  I've been using this 
method with a good degree of success, even with cels that
have large sticking areas.  

This does cause minor damage to the sketch.  The one thing 
to be wary of is that this method has the potential to tear
your trace lines off. That happened to one of my garbage 
cels(and not my favorites! Whew!) so beware of that
potential danger.

It seems like none of these methods posted are perfect. 
The hair dryer method can curl your cel(possibly damage the
paint) and the meticulous peeling method has it's dangers too.

It's up to you to decide of the drawing is really worth saving.