Mentally draw a tic-tac-toe grid onto the frame dividing it into thirds vertically and horizontally. The four vertices of the lines (or the four corners of the center square, depending on how much Hollywood Squares you've watched) are where you put your center of interest. (Unless you're shooting headshots, which is almost never a good idea anyeay, but more on that later.)

If you can't decide whether to use the top or bottom (or left or right) third for your subject, because either choice leaves too much of the frame blank, you're not shooting tight enough.

For headshots, full-frontal looks like a mug shot. At least have their shoulders turned to one side or the other. Abuse of this guideline, however, leads to all the faux-"action" shots you see in coprorate literature. Most people can't walk this fine line very consistently. This is why we make professional photographers.