Until Retopoflow goes 2.8, I’ve been trying to do some retopology with snapping vertices to face and the mirror modifier. But I was wondering ↑ title ↑. But also keeps the vertices snapped to the face?
I don’t know about any ways to do this in sculpt mode, but you can do it with modifiers. Add a Smooth and a Shrinkwrap modifier. Smooth evens out the topology, Shrinkwrap keeps it close to your high poly. Just select your highpoly mesh as your target in the Shrinkwrap modifier’s settings.
The Sculpt Mode Features Blender branch has a tool that relaxes topology while keeping the shapes. You can find it somewhere in the middle of this page about the branch (video beneath “Other features” header):