The FAST Rule

F Focus: be sure the subject is in sharp focus.
A Aperture: set the aperture (f-stop) required to correctly expose the scene. Don't forget the aperture's effect on depth-of-field.
S Shutter: set the shutter speed required to correctly expose the scene. Is the camera on a tripod for long shutter speeds? Don't forget the effect of shutter speed on sharpness and its ability top freeze motion.
T Think! Have you forgotten something? Is this really a good shot? Okay, now shoot!