Following a couple of rules of thumb can help you compose photos that are more visually pleasing.
First, there’s the age-old rule of thirds, in which you divide the image rectangle into thirds and place your photo’s subject at or near one of the intersections of the resulting grid.
Place your photo’s subject at or near these intersections.
This composition technique yields images that are more visually dynamic. The Crop tool in iPhoto’s edit view makes it easy to crop according to the rule of thirds (page 53).
A second technique ...