Rule of Thirds
"When we take a photograph with the main subject smack dab in the middle of the image, the viewer tends to only see the middle of the photo and their eyes stay fixed there without scanning the rest of the image. Here is the 'rule of thirds': move the subject off center to one of the lines of an imaginary 'tic-tac-toe' pattern on the scene. This gets viewers to look around the photograph. Whenever possible, move the subject out of the 'middle square' of that tic-tac-toe board."