Just because a digital camera allows you to reshoot photos without wasting film doesn't mean that you don't need to think before you take a photo.

Is the horizon straight? Many of us have a tendency to tilt the camera slightly, especially if we wear glasses when trying to peer through a tiny viewfinder. This is one occasion when taking the picture while looking in the LCD display instead of the viewfinder might be helpful, despite the greater risk of blur.

What is the subject matter? If you want to take a picture of a person, it is usually the face and eyes that say most about that person. Move in closer and capture their expression. If it is a land- or sea- scape, what is the most important element? The land (sea) or sky? Don't place the horizon dead in the middle. Give either the land (sea) two thirds or one third, depending on its importance.

Talking of thirds, there is a much-quoted rule used for teaching basic composition. Divide the picture into thirds both horizontally and vertically. Place objects of interest where the imaginary lines dividing the picture would intersect. Avoid placing the main subject matter too centrally. Experiment and take a series of shots of the same scene framing it using the rule of thirds to alter emphasis and interest. Which looks best? What do other people think?

For some excellent and very professional advice on photo-composition, clearly illustrated with photos and diagrams, try http://www.risd.org/schools/phs/clubs/ponyexpr/photoj/composi/index.htm. Another very good site for advice on composition is http://www.azuswebworks.com/photography/ph_comp.html. Between these two sites you can learn a lot about photo composition.