Adding subtle layers of variety is the key, I guess. Blending by different sized fractals and partly covering surface layers until you have enough color and displacement variety. I've added some voronoi cracks, for instance, but blended most out by a fractal, so you see it only sparsely (look closely). I think lots of those barely visible variation makes a 'CG-kind of' image more real. And TG is perfect for that.