Here is what I do:
1).  Replace bag or frame it.
2).  Remove all staples.
3).  Leave it in its original state.  It won't get
any dirtier as long as it is in a clean cel bag/frame.
4).  Try to scan it in the bag if the bag is nice
and clean.  Less handling, less damage.
5).  I put cel in cel bag by itself, put the
background in cel bag with the sketch behind the
background, then place the cel inside the cel bag
with the background in order to give it an
original appearance without having to worry about
the cel and background getting stuck.
6).  If it is removeable, remove it.  I will
NEVER, EVER, EVER buy a cel with a stuck sketch. 
I HATE THEM, HATE THEM, HATE THEM!  And I always
plan to frame my cels.  A cel is useless to me
unless I can frame it.

Cheers,

Star Phoenix