Out of all the steps involved in taking a photograph, the one that creates most impact is correct composition. It is also the one that is the most fun and where you are likely to have the greatest control. It follows, then, that getting composition right in the viewfinder at the time of shooting will yield the best results with regard to the final image.
So, how
 can you ensure great composition and a great image?
fill 
 the viewfinder: if
 your subject is too far away, get closer; if it is too near, move further back.
 Alternatively, use a zoom lens to alter the relative size of the subject in the
 viewfinder
 avoid
 empty space: look around in the viewfinder and only choose to shoot when you are certain all
 the elements you require are in view
don't
 just look at the subject:
 check the background and the foreground. Are there unsightly objects or distractions?
 If so, what can you do about it? Moving your subject might be an option.
 Otherwise you can either remove the distractions or shoot from a different
 angle
check
 your viewfinder: next
 time you have your camera to your eye, look around at the whole field of view
 in the viewfinder. Look at the edges, the top and bottom and both sides. It is
 so easy just to focus in on the subject without being aware of the huge spaces
 around it
isolate
 your subject:
 anything near to the subject will create a distraction. You can isolate it in a
 number of ways: move the subject or move the distractions; use a larger
 aperture to throw the foreground an background out of focus; wait until the
 surroundings get clearer (e.g. when taking pictures in crowds); get in close
 turn
 your camera: most
 cameras have a rectangular screen and picture taking capability. Most shots
 will be in landscape format (with the long sides of the image at the top and
 bottom resulting in a postcard-shaped image). If you turn the camera 90 degrees
 you then have portrait view. Try using this for some of your
 subjects and see what difference it makes
Many of
 these steps may seem simple, but you will be surprised how many photographers
 ignore basic rules. By thinking about your composition when you take your
 photography, you will enhance the potential of the final image and improve your
 photographic skills.
Eric Hartwell runs the photography resource site http://www.theshutter.co.uk and the associated discussion forums as well as the regular weblog at http://thephotographysite.blogspot.com