Hi, I've heard a couple of things, one is to use old cow dung (might be difficult) the other is to use milk I think (or was that lichens?). I guess the natural way is to ensure the rocks are a bit dirty with soil, keep them moist and in the shade. You might try breaking off little bits and put into crevices in the rock.
I am afraid my technique isn't too popular, but here is how I did it. I took rocks (mostly fossils) out to my family's farm and laid them along a shaded trail. Then I forgot about them for 10 - 20 years. Viola - moss covered rocks! I've created a moss garden from them in the back yard. Of course, the fossils make the rocks interesting with or without the moss!
