We all remember from English class the rule of Subject -- Verb Agreement, which means you wouldn't say "That book are great…" but rather "That book is great." The same rule can be applied to photography, if we look at how the subject of the photo relates to the framing of the picture, what I call Subject -- Frame Agreement.
The most common incident where we find this error is in distance. Being too far away from the subject does not make a good photograph. This does not mean that every picture has to be a real close headshot, it just means you have to pay attention to the background. Usually it is nice to include some of the surroundings, to give the image context, but you have to make a conscious decision that the background you are including is important.
When on vacation, there is often a major landmark that inevitably everyone must take a picture by. How many photos have you seen of a tiny person under the Eiffel tower, and not even all of the tower appears in the picture? A better shot might be taken from down the street, where you can easily fit the whole tower in the frame. Position the subject anywhere from 5 - 10 feet away from you, with the landmark in the background. Because your subject is closer, you can recognize his or her face and therefore have the evidence you need that yes, you were there.
Another instance in which we find subject -- frame disagreement is by taking a vertical image horizontally. Meaning the subject: a person, tower, statue, etc is a vertically oriented object, yet the picture is taken horizontally. Therefore, the picture ends up either being too far away, or cutting off part of the subject. Cameras may appear designed for horizontal pictures and many people just forget that they can actually rotate the camera, 10, 45, even 90 degrees.
In the image seen to the left, the distance is not bad, in order to show the harbor background behind the couple. However, a vertical image might have worked better, showing all of them and not cutting off any feet.
Ultimately, the trick is to be conscious of your picture-taking. Especially with digital cameras we tend to just shoot and not think. Spend another 30 seconds to make sure you have proper subject -- frame agreement, and you'll see great improvement in your photos.