I have tried this techniques with a sketch that 
was badly stuck to the cel.  I left the cel in 
the freezer for a few hours, not a few minutes 
like some posts have mentioned.  I put the cel 
directly in the freezer without it's cel bag.  I 
pull the sketch off slowly & it came of with 
eaze.  The only thing was, there was a little 
paint on the sketch.  This didn't bother me 
since it was on the back of the sketch. 

I'm going to try it with  layered cels that are 
stuck together.  I hoping that it works just as 
well.  I would suggest trying it with a less 
expensive cel first.  If it works out really 
well, then try it with the expensive one.

As to the hair dryer technique, I personally 
wouldn't try it.  I'd be afraid that I would 
melt the paint.

I hope this helps & good luck unsticking your 
cels.

Anime Krazy