I've started using a tool called retopoflow by cgcookie to create low-poly mesh that snaps to the surface of the high-poly sculpt. In one of the tutorial videos, they mention that they aren't going for tons of detail because "[you'd use a displacement modifier for that]". So, if you have a high-poly sculpt and a low-poly re-topology, how would you go about generating such a displacement map?
Here's a screen shot. As you can see, the re-topo mesh is much less detailed than the sculpt. The narrator says that he would bring in a displacement map for the details left out in the re-topology.