Rather than placing your subject in the middle of the screen, place it on one side and ensure something is in the background that fills the rest of the image. This can be really effective if the background has the same theme. For example, if photographing a football player, put the field goal posts in the background.
The Rule of Thirds
One of the most popular rules in photography is the "Rule of Thirds". It is a simple rule that can add to your photos. Simply, divide the image into thirds both horizontally and vertically. When composing your shot, place important elements either along these lines, or where the lines join together. - NOT at the centre of the frame.
It's a very simple rule to follow and will result in a nicely balanced image. It also helps get rid of the bulls eye effect. ie: placing the subject dead center with a lot of empty space around them.
The Rule of Thirds is more of a guideline and sometimes you will find a better image when you break the rule.