I think I've read about people *ironing* them out, when
there is no paint on the crease itself. But I have never
tried such a thing. I only have one cel with damage to the
acetate and it is much too much a beloved cel for me to
risk doing anything to worsen the situation. (Also, the
damage isn't bad enough to encourage me to try anything.)

I suppose on the simplest level, you could put the cel in a
large book for a few months and see if the crease
straightened somewhat, or at least flattened out again. But
I'd be careful if the crease is on a painted area.

Many Sharp Smiles,
--Drac