I've twice used a technique posted on the forum a 
while back by Chad Kawakami (of the Internet Guide 
to Anime Cels).  His post is here:

 http://
www.eu.animanga.com/db/celsarchive2/cmd=thread&aid=
439994834307231275&template=show.html&query=ID=
'439994834307231275'&var=tstart=1&var=tnb=30

The only thing I did differently is that I didn't 
get the sketch wet.  Instead, I picked the paint 
chip off the sketch (very gently) with a very fine 
paintbrush.  I moistened the chip and the area 
where it was to go, and nudged it into place with 
the brush.  Then I put a cel bag over the back of 
the cel and stuck a smooth paper weight on top of 
the affected area for a few days (dunno if that did 
anything or not).  Obviously if the chip is very 
strongly stuck to the sketch, this won't work.

Your Adhesion May Vary, I'm not responsible if you 
do this and your cel explodes, etc., etc.  :-)

--Atiya