You should check out Peris' great tutorial here: http://www.brameulaers.com/tutorials/ge ... orial.html
I'm not sure if it exists in Zbrush, but in Mudbox there's a brush called scrape that cuts out hard edges, which works really good for stone. It usually works best to start with a beveled cube for a brick, with enough edge loops to keep the sharp edges when it's smoothed. That way you can work away from perfect edges, rather than towards, which is fairly hard in a sculpting program.