I don't have that much experience with sculpting rock formations as I've just started using ZBrush 4 days ago, but I feel I've gotten some pretty good results with just a few simple techniques.
Here's how my rock formation was on Friday after I had just started using ZBrush.
And here's how it was Saturday after I experimented with a few brush suggestions from some fellow Polycounters.
Here's a general step-by-step of how I made that.
1. Start with a cube in Max that has 8 splits along each axis in Max to import into ZBrush. I find it's better to make the base cube in Max as it will be all quads compared to ZBrush's standard cube that has triangles on 2 sides.
2. Once you have your cube in ZBrush, start smoothing and pulling out using the Standard Brush and Clay or Clay Buildup while adding subdivision levels when you need more to manipulate.
3. Once you get a good basic form for the shape of the rock start adding and subtracting medium shapes/formations using Mallet Fast.
4. Go over the rock using Polish to flatten out areas and give the rock more hard edges.
5. Using the standard brush with Drag Rectangle and an alpha derived from a cliff/rock wall from CGtextures give the rock some light detailing.