Nothing works 100% of the time. One of the 
safest methods, if you are willing to sacrifice 
the sketch, is to use a damp sponge to soak the 
sketch and peel it off... but this will ruin the 
sketch in the process. I've had some luck with 
the freezer method. Basically, put your cel in 
the freezer for awhile on a piece of cardboard 
or something, take it out, and use a piece of 
paper to slide it between the cel or sketch -- 
but don't force it! After getting a few cm free, 
put it back in the freezer and repeat. It took 
me over a dozen tries at this to get my Nataku X 
cel free from its sketch.  Be warned, though... 
sometimes this won't work and you are risking 
separating the paint from the cel. Just be 
careful not to force it!

Good luck with whatever you decide.

- Karen