The Eyes Have It!

"The Eyes are the Window to the Soul"
The most important element of a portrait is the subject's eyes. Eyes reveal the emotions, thoughts, health and character of a person, so it's vital you capture them accurately.
Always focus your camera on your subject's eyes.
Unless you're doing an artistic portrait that focuses on some other part of the body it's always a great idea to make sure the eyes are as sharp as they can get. If something else in the photo is sharp besides the eyes, it tends to look like a focusing mistake.

Most cameras have a focusing point in the center of the frame. If you hold the shutter button (the one that takes the picture) down half way the camera will focus on whatever the focus point is on. Once you've focused on the eyes, keep the shutter pressed half way and recompose your shot the way you want it and press the button all the way to capture the shot.

Many cameras (such as the Canon SD850) have a "Face Finder" feature that detects faces in the frame and focuses on them. This is a handy tool since it saves you from having to focus and recompose your shot every time.

Lighting the Eyes
When you have highlights in the eyes it makes your subject come to life. On the other hand, when there's no light in their eyes their portrait can feel dull and lifeless. To make sure your subject has highlights in their eyes, turn their face toward the light source. If there's nothing shining light or bouncing it onto the front of their face, they won't have a twinkle in their eyes.

Some final thoughts on eyes...
If you have a cooperative subject, having the subject look just above the lens (about the top of your head) makes the eyes seem more open and alert.

Whenever photographing a woman, I recommend makeup for the eyes. At a minimum I like to see some mascara, which really enhances the portrait immensely.