I was recently talking to some crafty friends about taking portraits and in my opinion the eyes have it! Focus on the eyes and you can get away with the rest of the face being a little soft. Try and capture the reflection of the light in the eyes, also know as catch lights as this adds life to your subject. The best time of day to take a photo is during the golden hour, just before the sun sets or just after it rises. The photos below were taken in large open interior spaces with loads of light for professional headshots. However we you can do this with your own images.
How to do this at home:
In this example below I asked my daughter to stand just inside the door where she would be in shade, and got her to face the light. As you can see in the close up the lights were caught in her eyes (as well as myself).
If you are out and about, try placing your subject just under the shade of a tree facing the light. You will need to adjust the contrast and remove any colour casts using your photo software but you should have the same effect, giving the eyes some beautiful life.