My offerings:

I.  Patience.  Rendering is just about the most frustrating thing on earth. Patience.

II. Don't stop modelling the details.  Those are what make people gasp at an image, even if the image is supposed to look clean.

III. Texture with object cubic mapping.

IV. Patience.

V. When not doing III, use photo textures and height maps extensively.

Cheers,
-MG