Oooh. I did a tutorial on creating a concrete retaining wall. Not really sure where to find it. Mine was made with cardboard and coated in plasterer's joint compound for texture and then painted with clear glazes to give colour.
I've known others to coat in a thick PVA glue for texture.
Harry had it linked to the web page at one stage. I'll have a rummage and see if I can find it.