I've unstuck sketches many times myself, although
I wouldn't consider myself a professional.

My personal rules of thumb:

- If the paint is already peeling onto the paper,
it is usually better to leave it be. If it is only
peeling from one side, you might be able to remove
it by using the paper method described below from
the opposite direction of the peel, but this is risky.

- If it is stuck to a blank or worthless piece of
paper, not a sketch, it is usually best to take it
off by soaking the paper with water using a sponge
rag, or wet paper towel, then peeling the soaked
paper off -- but this will always ruin the paper

- In other cases, I put the cel in the freezer for
awhile to loosen the paint's hold on the paper.  I
then take a blank piece of paper (laserjet paper
is great) and slide it between the cel and paper.
Paper is sharp enough to cut the bonds, but not
sharp enough to cut the paint or cel. After 30
seconds to a minute of this, put it back in the
freezer. This can take awhile, but I've had a lot
of success with it. Be sure to use fresh cutting
paper after a bit of use, it gets dull fast.

Hope this helps.

- Karen