Lines are top element of design, so important, without lines nothing exist, even us are all defined by lines, very powerful not only to lead the viewer in a frame but to define shape and tension, it can make your image serene using symmetric horizontal or vertical lines or a dynamic looks and feel using angle diagonal lines.
|5D Markiii, f16, 1/2sec, 24mm, ISO 200, HDR, look at all the line the X, his legs, pyjama, all elements of design.|
Lines are very powerful elements that add dynamic impact to a photograph , depending where they are located in the frame, at the edge, splitting the frame in half in diagonal, zigzagging, curving, when you think about lines don't only think about straight line, straight lines are in fact very important, but a good exercise is to look at all lines in your image. One way to see them better is too turn you camera to black and white, if you shoot raw and JPEG you'll get both images, color and B&W, it's a great way to see the lines, to practice placing them into your frame. Look for patterns that can add to your image.
|Lensbaby soft optic f4, 1/1250sec, ISO 200, diagonal lines|
Even you frame itself is created by lines, it's a rectangle, and depending if you shot landscape of portrait that will change the impact of the shot, always try to shoot a scene both ways, and look at the lines, how they impact the result of the image, by example if you shoot the sea and landscape most of the time the horizontal framing will work best and will convey a message of stability, doesn't mean that you have to always shoot landscape horizontally, and you should try to always shoot both ways. Portrait/vertical framing will work very well for trees, building, people, with this framing all the vertical lines can be accentuate and bring strength and power.
|5D Markiii, f9, 1/60sec, ISO 200, 24mm, strong vertical line, triangle, vanishing point, all strong elements of design|
Don't forget to brake the rules, try something different cut the frame in half, with horizontal, vertical or diagonal lines, practice different points of view, lead your viewer in and out of the frame, have fun, if you are using digital you can afford it, the only suggestion if you split the frame or put a line at the edge of the frame, is to put your line as straight as the frame itself.
|5D Markii, f4, 1/160sec, ISO 200, 85mm, vertical lines in horizontal frame|
Many studies have been done into how people read the frame, from left to right, it's not only a reading method but a way that movies were and are still done, watch a program on tv, and the camera move from left to right, that's why most people view image from left to right, so a diagonal line starting at the bottom left and moving to the top right of an image can be quite useful and natural, but that doesn't mean that you have to stop there, try bottom right to top left, go corner to corner, in different directions and intersections.
Have a look at my images, all of them are in black and white, easier to see the lines.