You can tell I used a large softbox to light this by the highlights in her eyes. The reflector opposite the softbox is less obvious. I placed it just close enough to keep the keep the shadows open but not so close that I would lose the three-dimensionality of the lighting.
Even though the bulk of the photos I shoot these days are candids and street photography, I can shoot a professional quality portrait when the need arises. In the even that you get called upon to shoot one yourself--for a friend's LinkedIn page or website for example--or you'd simply like to improve your portrait skills, here are a few tips that should come in handy.
1. Think about the background first.
In a classic headshot your subject is in the foreground while the background is, well, somewhere in back. It's generally best to choose a background that has neutral color, tone, or pattern; one that won't visually merge with your subject, yet not so bright and colorful that it becomes a distraction. A pure white background can work as long as it's not so bright that it causes lens flare.
Also think about the distance between your subject and the background. The greater the separation between the subject and background the greater your ability to use depth-of-field to control how in focus the background will be. A telephoto lens and a wide aperture can do wonders to blur away a distracting background. Separation between the subject and background also allows you the option of lighting or shading the background separate from the subject and thereby prevent tonal mergers.
2. Start with one light, preferably large and close.
A common mistake portait beginners make is to overlight the subject. There are too many lights coming from too many different directions and most of them wrong. I find it's a lot easier to get the light right when you start with one light and then move either the light or the subject until the highlights and shadows look good to the eye. This obviously takes some judgement and the best position with vary with the subject. Generally speaking, you want the light off to one side or the other, otherwise the lighting will look flat and one-dimensional. You also want to raise it a bit, so there's a slight shadow under the nose.
The size of the light and how close it is to the subject will determine how soft and diffuse the light will appear. If I'm using a 42-inch diameter umbrella or a 48-inch wide softbox I generally place it no further away than 36-inches from the subject. If I'm a large, north-facing window, I place the subject as close as possible without getting the window into the photo.
Here the light source is smaller and the shadows darker. There is still some light filling in the shadows though, as you can see from the detail in her black shirt. As with the first photo, the background was a simple roll of white backdrop paper, lit to reproduce as a shade brighter than her skin.
3. Fill in the shadows to taste.
If you use one light you will have one shadow area. How dark it is will depend on the general reflectivity of the room or the area you're in. If you're shooting digital you have the luxury of being able to tell right away just how dark the shadows appear in a photograph. If the shadows are too dark, place a large white reflector, such as a piece of foamcore or matteboard, opposite your light source. The closer and/or brighter the reflector, the lighter the shadows will be. If you're looking for darker shadows, try a sheet of black foamcore or matteboard.
4. Be relaxed and relax your subject.
Remember that you're dealing with a human being, not an inanimate object with no insecurities and limitless patience. The more attention you pay to making your subject feel relaxed and confident, the more these feelings will show in their expressions. Even the most beautiful and "perfect" lighting will be wasted if your subject looks tense, confused, apprehensive, or annoyed. So keep the mood light. A few compliments, a few jokes, a few clear directions ("Turn your face toward the light a bit. Ah! That's it. Perfect.") and frequent clicks of the shutter when everything falls into place will do wonders for their confidence and yours.
Dozens of photographers have written entire books on the subject of how to light and pose portraits, so there's obviously more to learn than just these four tips. Nevertheless, if I were to boil what I've learned down to just four tips, these would be them. The rest is just practice, experience, knowing what you're look for, and paying close attention to what you see happening in front of you. If any of you have anything of value to add, either advisory or precautionary, feel free to add a short comment. None of us, myself included, knows so much that we can't learn a thing or two.