A basic rule of composition is to divide your frame into thirds and then place the subject at the intersection of the lines, or "sweet spots," allowing the eye to take in the entire scene.
Powerful images can also be created by placing your subject in the center, especially when there is symmetry and a strong point of reference, as in this image of children playing on a trampoline in South Africa. The energy of the jumping boy in the center is enhanced by the fact that the eye is drawn directly to him, while the children on either side form a pleasing frame.