Another thing that contributes to the vacant stare is that people often appear to forget that the eyebrows can be adjusted as well. Raise them. Lower them. Add worry to them. That adds to the realism of the look. Also, it seems to help if the eyes aren't perfectly parallel. In other words, if the figure is looking straight towards the camera, adjust them inwards, towards each other, just a few degrees. I'm not saying make them cross-eyed, just a few (2-3) degrees towards each other.

Before they made me they broke the mold!
http://home.roadrunner.com/~kflach/