I think there are a few key things to watch when trying to get renders as realistic as possible.  First off the included shaders with Carrara need tweaking.  I have yet to use one which didn't need adjusting.  Mostly I adjust the reflection, highlight, shininess and bump channels.  Sometimes I add a bit of translucency to certain shaders as well.  The example of the cup and saucer in the first image on this thread looks fairly straightforward.  Also keep the lighting as realistic as possible.  Not a lot of beginners know this but our sun is not white.  It has a faint yellow tinge to it.  Changing the sun light color just a tad, makes a huge difference in the final render.  Also even coffee mugs have a very very slight translucency to them.  Subsurface scattering also makes a huge difference.

People are another matter.  To get realism from a poser import, you need to make sure that it looks real in Poser first and then edit the shaders Carrara creates on import.  Adding a bit of bump here and some translucency there.

Basically picture in your minds eye what the real McCoy looks like and experiment until you get it right.