The rule of thirds..is a useful one. It says that you divide the image into a grid, jut like the one below. Then, when you're composing your shot, you place significant items along / up any of those lines. Horizons are easy, place on the lower line for an airy feel, on the upper line for a more closed-in feel.
Put your main subject somewhere where two lines cross and you'll boost its power, its dominance in the shot. Have a look at the following images.
In Firefox (others?), you can drag the grid and slide it on top of the images here (let go and it'll pop back). Look at what it tells you about where I've placed the various elements in these compositions. You'll notice things don't have to be exact—near the lines / crossing-points works too, I think you'll agree.
DistractionsWell, I haven't got any examples from my work for this heading! I'm always very careful with distractions (a bit of a pedant actually!). So what do I mean? Well there's a couple of things that fit here. (I'm assuming here you're past the point of shooting people with telegraph poles sticking out their heads!)
First Contrasting areas near the edge of the frame. For example, if you have mainly dark borders with a light blob next to the image edge, then the viewer's eyes will be pulled right to that blob and then likely right out the picture. Even if they're looking in the body of the image, as soon as their eyes get anywhere near that blob, bang, they'll be drawn right to it. Or maybe it's a tree branch sticking its nose in, or some other distraction intruding into the frame.
Second For example, one or two birds that are just dots in the sky (or maybe there's one or to spots of chewing gum on the pavement in a street scene). If the sky is uniform and those dots contrast against it, again the viewer's eyes will notice. But once their attention is drawn, there's nothing but dirty smudges to see. Sorry, they've got to go. A stray crisp bag, one corner poking into the scene from under a bush?—get rid of it.
SimplicityIf there's no good reason to include something in your shot, don't! Of course, when you're shooting from the hip because there's lots going on, well, the finer points go right out the window then anyway.
For other situations, those where you have more time and control, simplify, simplify and simplify again. Ok, again these are extreme examples, but they serve to illustrate.
HeightDon't always shoot standing up. Varying the height from which you shoot can have a huge impact on the final shot. Shooting kids / pets? Get down to their eye level, get below it if you can. Shoot a worm's eye view of a scene, getting as close to the ground as you can. Jump up on a wall to introduce foreshortening or just to get interesting lines.
In a nutshellPlace important elements along those powerful thirds' lines / intersections. Consider how to bring the viewer's eye into the picture, look for what can serve as lead in lines and / or foreground interest. Have a quick glance around for distractions and watch out for areas of high contrast along the frame edges.
Next stepsUse and abuse these rules the next time you get the chance. Try to get a feel for which compositional approach suits each scene you take a fancy to. Walk a few steps in each direction from the first viewpoint you naturally gravitate towards. See how any possible lead-in lines change their relationship with the scene as you move. Crouch down, step up high, look for foreground interest.
...and let me know how you get on, post a link in the comments. I'm looking forward to seeing how you get on.
comments / critique / feedback always welcome :)