I just finished writing a new composition lesson on the use of Leading Lines, Diagonals and Curves. The article can be found here on my personal website. It's the 4th in a series of articles or lessons about basic photographic composition.
The lesson discusses the use of lines to add a sense of movement to your images and to guide the eye to various elements within the photograph. It also discusses the use of other geometric entities such as triangles, arcs and curves and elements that can be used within an image to provide spatial separation, to frame and to direct the viewer's eye. I hope it's useful to anyone who has the interest and the time to read it. As always, feedback is welcome.