Composition or arranging your subjects in your scene so that they have a pleasing effect can be one of the hardest things to do or the easiest depending on your creative abilities and experience.
When I first started to take pictures I found that my images were lifeless and rather dull. They had no focus or shape and left the viewer wondering what they should be looking at.
I found the more pictures I took the more my eye became trained in seeing the image through the viewfinder and positioning subjects became instinctive rather than something I had to worry about. It really is practice that makes perfect and this video illustrates perfectly how by looking at others work; you can train your own eye to compose an interesting and arresting image.
Even though this video is 11 minutes long it really is worth viewing all the way through.
This video from 'fubdog' shows how compose your photographs effectivly.