Rule of Thirds - Improve your photo taking by splitting your frame into thirds both vertically and horizontal. Place the main subject along this imaginary lines.

You will also need to remember that if you are photographing something moving then you should give it space to move into, rather than space behind it. For example, if you are photographing a boat running from the left then place the horse in either the bottom left or top left line intersection on the grid, then have most of the picture showing water and sky to run into, a picture like this shows freedom and movement and is interesting and pleasing to look at.

Other examples shows more complex scenes rendered with rule of thirds in mind. The thing to observe is not to always center the image. Place elements like horizon, mass of an object or other distinguishing traits along these imaginary lines.