The third rule of composition is "Leading Lines". The idea is to use defined lines in an image to make the viewers eye follow along from one area of the image to another. The line can be anything, roads, trails, fence line, rivers, train tracks... These are some of the more obvious ones. Try useing shorelines, tree limbs, a cresting wave, a crack in a rock, or maybe a row in a beautiful garden to carry the viewers eye through the shot.
Leading lines work very well portraying depth in your image. A diagonal line across your image will make the viewer feel distance. In the night shot above taken in Reed Bingham State Park in GA you can tell exactly how much distance is between each of the trees because the line gets thiner as it gets farther away. As the line weaves in and out of the pines it adds depth.