Victoria,
I'm sure there's more than one way to do this, but I like to use colors to mark which photos have been uploaded to my stock photo site, which ones still need to be sifted through, and such. In some cases I also use color to mark if I have uploaded a photo to a separate site. I realize all of this could be done within collections, or even within a part of the meta data. It's just that I am a visual person, and so having the option to mark everything with color tells me what I need to know more quickly. Honestly, it would be nice to be able to mark one photo with more than one color... Perhaps as sizable dots in one corner. That way if I upload the same photo to more than one site I'll know.
Rick