Thanks to all who have contributed, i myself have made many of the same mistakes mentioned here already, and am learning from them, but to read about it from others really hammers it home.
something i have noticed, and am guilty off it myself: skin smoothing,
there is such a thing as too much. Sometimes natural skin( Obviously depending on the skill of the MUA) can look really great!
and if you do the face, don't leave a line around the chin ( nifty little feature in CS5 is surface blur.) Just not too much.