In trying to sculpt a long trunk-like structure for my Helbug, I hit on the very simple technique below. Mostly, I wanted what looked like a leathery trunk dangling down from my Helbug (you know, to slurp up all that Vector Stricken and Bale Flamer'ed loyalist paste).
I decided I needed a bunch of randomly near-parallel lines to impress on the surface of some Green Stuff. After sitting and stewing on it a bit, it struck me like a brick of fried gold: paintbrush bristles!
So, you start with a smooth surface of GS.
Once again I didn't think to take any pictures until after I finished the parts I wanted to texture. So, I busted out some more GS, above, so I could at least show you how simple this is.
Next, find a paintbrush with a wide set of bristles. Wide brushes are best because it shields the GS from your fingerprints. Thicker bristles give you a more aggressive texture.
Then, all you do is randomly, but gently, impress the side of the bristles into the GS. Just keep going until you have something you like. Use a clean brush and watch out for bristles that get left behind.
When all is done, you end up with something like this...
And this (a truncated trunk)...
For the above sculpt, I shaped the end of the trunk around the cap of a bottle of Vallejo paint. Hopefully, when it cures, I can just twist it off.
And this, too...
Now, as long as the paint doesn't fill in all that great detail, I'll be golden.