Do you have any tips on how to create texture on skin when digitally painting?
You've answered your own question - add texture!
Add some texture layers, just be subtle about it. Keep the opacity low, paint over it, maybe add another texture. Layer those suckers up! Just experiment a little bit and see what works.
Also, don't use an airbrush - or any kind of soft fuzzy wuzzy brush. They'll suck the life right out of your skin, just practice and you'll get nice blending whilst still retaining texture.
This is a very nice way of adding some grain: http://adonihs.deviantart.com/art/THE-GRAIN-TUTORIAL-157135405?q=sort%3Atime+favby%3Asnatti&qo=1&offset=220