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.

Found this thread just about that :)

Vera

Here is a link that might be useful: Growing Moss with a moss milkshake

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!