Heat = very bad.  Don't try it unless the cels 
are expendable.

Weight on top of the creased cels = cheap 
solution but probably not that effective.  I've 
had a few cels bent on the corners and have 
placed a ton of other books on top in the hopes 
of fixing them.  Unfortunately, the creases 
never go away under this method ... but at least 
the cels are usually flattened nicely (over 
time) and most times the creases are diminished 
enough to make the cels presentable again.

Bent corners = use a matte to hide them and 
forget about them.

Best bet = consult with a professional cel 
restorer.  These places have been mentioned in 
older threads:

http://www.srlabs.com/index.html

http://www.vintageip.com/

I don't think anyone has ever found a sure-fire 
solution to creasing in the cel.  Best of luck.