I've tried the freezer method as well, although 
never overnight.  So far, I've had pretty good 
success -- one of the things that I tend to do is 
to hold the cel by the sketch and try to let 
gravity do most of the work in separation by 
holding it at an angle (kind of difficult to 
explain the exact method, hopefully you get the 
idea).

I may try the overnight method on the one cel 
that I wasn't able to separate the last time I 
tried (cel fills almost the entire sheet of 
acetate and almost all of it is stuck).