I had the exact same problem as you.  It's actually very difficult to find a decent skin tutorial!

First problem you stated was turning clean plastic-looking skin into realistic-looking skin.  The best way for that is to find a decnt texture...;)  No, seriously, here's what I did to the skin to produce a more realistic look for this image:

http://www.renderosity.com/mod/gallery/media/folder_128/file_1272516.jpg

Now, I work with Daz|Studio, but as the main work happens in photoshop, it can be used for any 3d program.  It'll also work with any image program that supports layers and blending modes.

Daz|Studio work:
The figure is V3 from Daz with an Aiko pose applied, V3's bonus texture (not the default one, the other one), and a few soft lights scattered about. She was rendered pretty quickly, then whisked over to Photoshop for the real work to begin.

Photoshop work:

First I fixed the places where the pose had creased V3 in odd ways (using a small soft smudge brush), then I took the smudge brush and ran it over her skin, blending it all together and providing a soft base. On another layer I repainted all V3's shadows with a greeny-coloured soft brush, then set a gaussian blur on it (about 10.0) set it to overlay and fiddled with the opacity and fill settings. Then I did the same for the highlights on a separate layer with white.  (gaussian blur and overlay again) I did the same for the shadows under the eyes and on the eyeball with black, then flattened the image. I then duplicated the final layer, did another guassian blur, and set that final one to overlay. I then realised it was much too orange, so I flattened the image and applied auto levels and auto colour to help lift it.

That gives you the softer look to the skin.  If you aren't after as much softness, lower the gaussian amount.  As for adding dirt streaks, or cuts etc, then I suggest (for a very simple and easy method ;) ) finding a picture of dirty ground, opening it in photoshop, and using the clone stamp tool to draw on the dirt you need.  I know you can get some very good blood brushes from deviantart (here's one for example: http://www.deviantart.com/deviation/19982524/ ) - just download and install!  (Instructions for installation included in the link, and also tips on creating the blood effect).  You will need to download it to see all the brushes.  now remember, brushes are greyscale, so in order to add the right colour to the blood, you'll need to follow this tutorial: http://ic1.deviantart.com/fs7/f/2005/205/a/5/Blood_Texture.jpg

Hope that helps!!