When you make a portrait, especially a headshot, shoot slightly above the eyes level. It does not need to be a lot higher, and you certainly need to avoid being lower than the eyes of your subject. This tip is especially useful if the person in front of your lens is overweight. 

The photo on the left has been shot slightly below the eyes level and the one on the right, slightly above. You can see the differences at the neck. On the right, the neck line is better defined. It eliminates or decreases the double chin effect. You can also ask the person to protrude the head a little.