This is pretty close to how the original, unprocessed Raw image looked. I just reduced the clarity (microcontrast) and color saturation a bit. Most of his expressions looked like this one. He's actually a very amiable and thoughtful young man who was trying to look "hard" for the camera. Canon EOS 60D with 85mm f/1.8 EF at f/11.
Portraits are the sort of thing that, once you've mastered the basic technique--which isn't particularly difficult as techniques go--the challenge becomes less about getting a portrait that's in focus and well lit and more about producing a portrait that has staying power. By "staying power" I mean an image's ability to maintain a viewer's interest over repeated viewings.
It's harder to do than you might think. The average portrait looks like the ones we see in countless school and college yearbooks: competent but generic and unimaginative. A great portait, on the other hand, offers insight into the subject's personality and state of mind even if you've never met them and don't know who they are.
I can't claim to have any exclusive secrets to achieving great portraits every time. I can, however, offer a few tips you may find helpful:
1. Pay attention to your subject.
Many photographers focus far too much on their lighting and not enough on their subject. A subject who senses this will naturally tend to withdraw or become bored or annoyed. Keep your lighting simple and flexible enough that your subject is the center of attention.
There's more to paying attention than simply staring at someone. You should try to observe and intuit how they feel. Are they nervous, insecure, distracted, fearful? Are they trying to project a certain image? Think about what you actually see in front you rather than what you or your subject might like to see. It could make for a far better portrait than what you originally had in mind.
3. Lure rather than chase.
Rather than try to manipulate and coax your subject into making a particular type of expression or pose, try to opposite approach: Calmly wait for them to let their guard down and make an expression or gesture that is characteristically their own. It may be the subtle shift of an eyebrow or a tilt of the head. If you're paying attention you'll know it when you see it and you'll release the shutter the instant you do.
4. Post-process to enhance, not exaggerate.
Chances are that even if you have your lighting and camera set up perfectly, you'll have to post-process your final picks to adjust their clarity, sharpness, color, and tonality. With portraits of females you'll generally want to "take the edge off" by muting the contrast and color saturation and smoothing out skin blemishes. You'll want to do the opposite with men. Take it easy though: If you go too far in either direction your subjects will either look plastic or as if they were hewn out of granite.
Whole books have been written on the subject of portraiture, so the list above is by no means all you need to know. It will make your portraits better though; that, and needless to say, plenty of practice.