For whatever reason, many of the tricks mentioned here are not common knowledge. It's most likely that there are tricks that are commonly known that have not percolated over to me yet. My apologies if I have overlooked anyone. Normally if a surface is lit by only ambient light, its shading is pretty crummy. For example, a non-reflective cube totally in shadow will have all of its faces shaded the exact same shade.
All edges disappear and the cube becomes a hexagonal blob.
The light at the eye gives the cube definition. However, this trick can lead to various artifacts, e.g. there will always be a highlight near the center of every specular sphere in the image. There are any number of efficiency schemes out there, including Bounding Volume Hierarchy, Octree, Grid, and 5D Ray Classification. See Jim Arvo's section of the book for an excellent overview of all of these. The most important conclusion is that any efficiency scheme is better than none.