It's not always convenient or economical to have a professional portrait photographer at your side. For those moments when you have to be your own photographer, keep in mind that the most important thing in a portrait (or any photograph for that matter) is light. In fact when you think about it, in any photograph you take, all you are doing is capturing areas of light and shadow that create an image.
When you break light down into its components, one of the most important things that a portrait photographer thinks about is the DIRECTION of light. Just keep in mind that there is no such thing as a "bad" direction. You choose the direction of light depending on what you want to say with your portrait. Using light in this way is one of the tools I use to transform a photograph.
Light from behind the subject can be used to create some interesting effects. If you expose for the backlight and underexpose the subject, you create a silhouette. If you expose for the subject, the backlight brightens even further, and you can create a beautiful golden rim around the hair and shoulders. Of course, the backlight in your portrait photograph can come in at many different angles and you can adjust your exposure to get anything from a black silhouette to a completely blown out background. Adjust the direction your subject is facing and your exposure to transform that gorgeous portrait in your mind into one on your memory card.
What about light from the sides or front? Many of us tend to place our subjects so that light falls straight from the front. This can create a "flat" look across the face that eliminates or minimizes many imperfections. For a more dramatic look in your portrait, turn the subject so that the light comes a little more from the side. This will create some shadows in your photograph that can make it more interesting; this technique can sometimes be more flattering to a male subject.
For real drama, bring the light in directly from one side or the other, at 90 degrees to the nose.
I hope this helps you get more out of your portrait photography. There's nothing quite like capturing that vision in your imagination and making it real!