UV Mapping tutorial. Once you've retopo'd and created UVs, subdivide it until you can project all the original details (it's good to "Project All" at every division along the way to keep the whole shape from having to change much), then go back to the division level you want to export and in the Tools Palette is a "Normal Map" button. Here you can generate a normal map. ZBrush will generate the normal map for the current subdivision level based on the highest subdivision level.

You can generate normal maps for different division levels. You can also generate an AO map by masking by AO and then doing a colour fill. This way it is very easy to generate multiple LoD levels and appropriate mapping for each one.

The UV Mapper doesn't give much freedom, but usually I prefer its fast workflow over the great freedom but long work of other tools. Also, generating LOD's this way doesn't give you as much control over the higher LoD levels, but it is fantastic for workflow. For me, it's exactly what I want.

Jibb