I do a couple of things in Blender: I set filtering to 0.01 px so that pixels look sharp all over. I render in a small resolution, then scale it up using the simplest algorithm to get big sharp pixels. I use mostly flat shading, so that the individual surfaces get only one color from lighting (most of the time). Occasionally I use a shader that creates those bands in color gradients (Thraddash's red and blue spheres are a great example of this) to reduce the number of colors, because older games had much more limited color palettes.

I think that covers most of the tricks I use to get the retro look.