Starkdog is correct, the most important bit is good neutrally lit photos. Get some from.. http://www.3d.sk/  or a merchant here.

Then it's all about copying a bit, pasting bit cloning, opacity and blending. It's about tone as well as colour.

One trick if your making 2nd skin clothing, say a robotic bra, is to use a vector package. Import a copy of the final skin map into something like coreldraw or the free Serif drawplus.

'Flesh' out the shape you want working on one side of the body only. Once it's all lined up OK on the actual map, Mirror it to the other side. Now export it back to Photoshop/Paintshop and flood fill wth a texture and apply any filters to suit.  

 

Pinky - you left the lens cap of your mind on again.