The problem I've found with just pulling the paper
off is sometimes the paper will remove lines and
even paint.  What I did for the first time recently
was put the whole shebang, cel and stuck sketch,
in the freezer for about 10-15 mintues.  I'd heard
of this method working for getting two stuck cels
apart and I decided to try it with sketches.  It 
worked pretty well and I managed to get the 
sketches off with no damage to the cel.  And, if 
you're really careful, you can have minimal damage
to the sketch if it's a matching one as was the 
case with one of my Duo Maxwell cels.  I don't
advise just trying to rip the sketch off because
it can harm the cel.  A little patience and cold
goes a long way. :)

Jen