A friend of mine gave me the following tip. When composing a photo, think of a matrix:
Try to get your subjects on those lines. For example, when making a picture of a beautiful sky, don't put the horizon in the middle of the photo. Try to get the clouds on the upper line and the horizon on the lower:
You can use the vertical lines as well. When clicking a photo of a man and a car, try to get both on the vertical lines. And again, think about the ground, can it be set on the lower line?
What's really nice is if you can put some perspective in the picture. This works best when you have something to the side. Use the matrix and snap the picture with a tree at the right: