Learn Photo Composition

No composition rule is universal.  No rule will work for every case.  Some clever people have noticed trends in photos that can be summarized as rules and I'll try to summarize some of those rules.

You can look at photos in two ways - either as a 2 dimensional plane with color, or as a three dimensional scene that has been flattened into a 2 dimensional photo.  Some types of photos (macro, long focal lengths or shallow depth of field) tend toward 2 dimensional interpretations and the 2D Composition rules seem to work well.  Other photos (mostly wide angle, deep depth of field) seem to require the strong three dimensional sense to really pop out to the user.  Of course no rule is hard - almost the opposite, simply following the rules often leads to just acceptable images.

Rules of Thirds: The rule of thirds is by far the best known composition rule.  If you divide the photo into thirds, place objects where the thirds cross.  Align horizons and edges to join the third lines.

Disappearing Lines into Corners: If there are strong lines in the scene, try to get them to disappear into the corner.   If the lines break into the center or the edge, it tends to divide the photo, but disappearing into a corner seems to make composition stronger.

This effect is often easily done with paths and railings.  It invites the viewer to walk into the photo or gives them a safe feeling.
Use "S" Curves
Apparently, they eye enjoys following "S" curves.  This rules seems to apply mostly to water, roads and the silhouettes of people.  It doesn't strictly need to be an "S" either - reverse it or even just a strong zig-zag can be enough.

This can be combined with the disappearing lines into the corner piece above to stronger effect.
Monotonous Content:
Some images would make killer jigsaw puzzles - they contain many examples of the same object but with deep detail allows the brain to dwell on and discover arbitrary parts of the photo.

For landscape and environment photos, there are additional strategies you can use to make your photos stand out.  These seem to tap into the brain's ability to perceive or reconstruct spacial relationships.

Using Layers of {Textures | Lighting} One of the easiest ways to recover three dimensions from a two dimensional rendering is to use layers of lighting or textures.  They eye can easily trace outlines and assign a depth sorting priority to the various pieces in the frame.

Using Silhouettes
Another aid to the eye seems to be the use of silhouettes.  Silhouettes allow you to form a stronger sense of scene depth because there is a colour becomes a function of depth. Dark Silhouettes at dusk are the easiest form of silhouette to form - just over expose the frame in a high contrast scene.

Using Depth of field to Guide the Viewer (Foreground Background Composition)
Sometimes a complex scene can be softened if the shallow depth of field is used.  If you have a trace that maps from the foreground to the background, that also can aid the effectiveness of the image.

The above rules are generic - could work for anything.  Different types of photos have different types of "rules" to be followed or broken.  For more specialized topics, see: