Well I prefer to try to write descriptions for
each of my cels.  I like to put some information
about what scene/episode that it came from if I
can.  Some people write very detailed descriptions
(cel #, scene/episode, layers and other info) but
sometimes I think it's better not to give out all
the specifics of a cel.

I used to not watermark my cels, but there are a
lot of people who will steal your images for good
or bad..and usually if they are taking your images
with out asking it's not for a good reason.  That
is completely up to you, it's extra work but I
prefer it.

If you have a lot of cels, I would recommend
categorizing them by show, some people will also
categorize by character if they have a lot of cels
from the same show.

Some people also have very big scans of their cels
and some have smaller ones, I guess that adds a
bit more protection from image stealing.

Well, I don't know if that helped any.
I guess it is all up to how you feel about certain
things.