One of the things I just discovered is really simple. If you look at just about any photograph one thing that is in every one is noise. Sometimes it is hard to see and sometimes it is obvious if you know what to look for. CG images don't have noise. A Terragen image was recently posted in the realism category and one of the postwork items was noise. Before I read that I wondered what the artist did that was special. Postwork a little bit of noise into an image and see if it looks more realistic, more like a photograph.