I'd pretty much agree with everything already 
said, but here's my personal advice:

Don't try to separate stuck cels. More than 
likely you're just going to end up doing damage 
to whatever is stuck together. Ex:  I have a 5 
layer Outlaw Star cel that 3 of the 5 layers are 
stuck & even though there is some dirt built up 
on the layers in between, I refuse to attempt 
separation for fear that I'd ruin the stuck 
layers.

As for cleaning: for the stuff that won't come 
off with a damp cloth:  use a Q-tip & some 
rubbing alcohol. Be VERY VERY careful because the 
alcohol can dissolve the cel paint.

I also use a brush to remove light dust from them 
before attempting any other kind of cleaning.

hope this helps!
--zerospace