Basically, I put the bent part of the cel between 
the last couple pages of a big book, then even 
maybe put a couple books on top of that. The only 
thing you have to be careful of is to make sure 
not to put the book on a soft surface (like a 
bed) where the weight will make it sink down, thus
bending the cel at the edges of the book. ^_^; 
It's also a good idea to have the cel in a cel 
bag when you do this to prevent any possible 
transfer of ink off of the pages of whatever book 
you're using.

As far as sticking goes, either the rice paper or 
pieces of cel bags should be fine to prevent 
that. ^_^

I would leave it like that for at least a week 
(depending on the severity of the bend/crease) 
and keep in mind, if it is a crease as opposed to 
a light bend, this won't eliminate the white line 
caused by the creasing.

Many Sharp Smiles,
--Drac