I have a very badly creased cel (thankfully, the image is still
sound) that I TRIED to uncrease by placing it under a heavy
book and leaving it while I was away at school.  After 6 months
of constant pressure, there was no real change in the crease
or the cel.  Of course, the crease on my cel had cracked somehow,
so there's really no way to put it back together.  I'd prolly
be better off cutting off the paint and slapping it on a new
piece of acetate, but I don't want to do that.  If your crease
isn't too severe, maybe the book thing would work for you, but
don't get your hopes up.

Alternatively, you could try a cel restorer.  I'm led to understand
they do good work.  Saiyan X's gallery has a link to a cel
restorer, if you want to look that up.

Good luck!
Alex