If the crease isn't across paint (God forbid!) 
this method worked okay for my Hiei cel that had 
a strange crease along one of the corners.

Put it on a glass top table with a heavy book or 
two on top of the crease (luckily I didn't have 
to touch the painted image with anything) and 
blowdry the crease from under the table on a low 
heat (if you have one of those heat focuser 
attachments, even better).  It isn't perfect, 
but it's far less noticeable now.  

Again, I'm hesitant to do anything to a painted 
image but I have my 'experiment' cel I could 
crease and try to straighten if you're 
desperate! (Poor guy, that's what he gets for 
being an anonymous freebie! ^_~)