that I found to work most of the time. When you 
freeze a cel the paint hardens and the sketch is 
easier to remove if you gently tug.  Make sure 
the cel is laying flat in the freezer and 
another piece of paper is under the stuck sketch 
(safety reasons). I usually leave it in there 
for almost an hour and if some of the sketch is 
still stuck when I try and remove it, I will 
freeze it again (after that it usually comes 
off).  I own no cel that is stuck to its sketch 
and there has been no damage done to the any of 
the cels either. 

But I have found some sketches to get damaged 
this way.  The pencil sometimes will stick to 
the cel paint and come off when you remove it 
(not the paper).  But in my opinion that is 
better then destroying the sketch all together 
like some people do by soaking it in water o_O

I do not believe this technique works on stuck 
cel layers though.