During my research I found this really good tutorial online http://www.paint-sculpt.com It shows a skin texture technique which is really effective. Other techniques that I have seen are similar but involve using a wrinkle tool which is basically a rod with several thin metal wires splayed out at the end. If you can imagine a mini garden rake then that is what it resembles. These tools can be made pretty easily or bought online. Another useful thing to use is a cat brush as the small metal bristles are also good for creating a wrinkle texture.