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"The 3D Look" is always a hot topic here on FM and tends to be somewhat subjective. Some will post images they believe are the most 3D on the planet and others will comment that they can't see it (same images). I suspect that differences in vision and color sensitivity among different people account for some of that, like some people who can see the images in Stereograms immediately and others that never manage to.
Some of these have already been mentioned but here is my list of things that can enhance the "3D-like" qualities of an image:
- Light vs. shadow (sculpting)
- Bright (closer) vs. dark (farther)
- More saturated (closer) vs. less saturated (farther)
- More contrast (closer) vs. less contrast (farther)
- Warmer colors (closer) vs. cooler colors (farther)
- Composition and leading lines
Some of the above that are "X vs. Y" can be reversed successfully in some images. Note that most of the above can be influenced both "in-camera" (via composition primarily) or in post-processing.
John Paul Caponigro has done some interesting writing and video on these effects, primarily as they relate to landscapes, but you can apply most of the techniques to people pics, too, if you have the time and patience.