What kind of realism are you looking for? The techniques vary for different scenes. Outdoors? Interiors? Portraits? Still life?

Modelling is important too. Very sharp edges, which are the simplest to create, are unrealistic. Most real world objects have slighly rounded or "chamfered" edges that catch the light.

Work hard on your textures - study the real-world surface you're trying to imitate and the way it interacts with light, and imitate those properties in your materials.

Also, irregularity - completely uniform objects or surfaces are rare in nature.

So many good scenes are spoiled by too much ambient light, which flattens the image, lessesns depth, makes them look like cartoons or drawings. To imitate real world scenes, make sure the light is right.

A good definition of the kind of ambient light that spoils realism is "light that doesn't seem to be coming from anywhere" i.e. your eye cannot explain its source. it doesn't look like direct light, it doesn't look like indirect light bounced off or refracted through objects.

If it's Interiors you're interested in, check out Chipp Walters' new "InteriorPak" - www.altuit.com