A few craggy rockfaces will add great realism to your hills, especially the stepped styrofoam type. This large hill has one face that is a huge, impenetrable crag face.
I usually add a few craggy faces to the stepped Styrofoam hills that I make and have some 'realistic' hills that have large cliff faces like the one in the photograph.
I achieve this effect by melting the Styrofoam face with a hot soldering iron that has a flat pyrography blade inserted. You can get the same effect with a hot knife or by melting the styrofoam with liquid poly cement or a similar solvent.
I then paint it dark grey and dry brush with light grey and finally white.
The scree at the bottom of the photograph is just gravel and small stones glued on with PVA and painted grey, then drybrushed with light grey and white.
Andy Skinner made this craggy hill in Epic scale using similar techniques. He says: this is a hill with steep cliffs, and a road winding around. It was made for use with Epic 40K. I'm not actually sure of its game effect yet, because I think it would take a couple of turns to make it up the road. But we'll see this Saturday. :)
The hill is made from 4 levels of blue insulation foam. I drew the top layer on foam and cut it out. Then I used that as a template for the second layer, expanding along one side where I wanted the road to wind around the cliffs formed by the top layer. Then the second layer was a template for the third, and the third for the fourth.
Although this allowed the layers to match up nicely, the general slope of each level was different. The edges of the layers match, but the foam cutter was angled differently for each piece. So I had to come up with a way to make the rocky face look good and cover the different layers. I originally planned to carve it the way I did the rocks on my dragon hill, but I decided that would take a long time and wouldn't cover the layer seams. So instead I coated the whole thing with joint compound. (I wish I mixed a lot of white glue into it, because I think that would have made it stronger--I'm a little worried about brittleness.) I put it on both the sides and the flat surfaces with a popsicle stick, and dabbed at it to get a rough look, very like the crag project above. I gave that a layer of spray primer, which also ate into places the foam was still exposed, giving some interesting holes and caves. I then painted it with gray wall paint (same used on dragon hill).
My wife had given me a can of Ralph Lauren textured paint (ooooh! We are getting posh on TerraGenesis - Gary). This is normally too expensive to suggest on a site for cheap terrain, but it was a mis-tint (they made the wrong color), and it cost her 75 cents! It is _perfect_ for painting sand onto surfaces. I mixed some of my gray paint with some of the sand-texture paint, so I wouldn't get too much contrast in sand that is supposed to have come from this same hill, and applied that to roads and the top flat surface. I gave the sides a wash of black (it didn't really get into the details the way a wash should, and mostly just darkened the surface overall), and two drybrushings of successively lighter gray.
I painted thinned glue onto part of the top surface, and onto various flat areas along the road or in little "pockets" on the cliff sides, and sprinkled flock onto those areas. I also used some Woodland Scenics foliage (that loose sheet of material) as vines.
Finally, I sprayed it with a semi-gloss sealer to protect it. I've heard that Testor's flat will dull down a glossy coat, so I'll give another layer of that when I get some.