This video shows my technique for creating clean edges. There are two techniques I use, both similar. One is to simply mask, in low subdiv, and pull the geometry over other geometry and then pinch to make the edges close so that after subdiv up they remain close.
The other is to turn off "subdiv smooth modifier" so when subdiv up levels the corners remain.
You will see both techniques in this video.
Making clean edges in Zbrush from Nick McGavern on Vimeo.