Make sure your horizon is horizontal! There's nothing worse than looking at a breathtaking shot with a horizon tilting to one side. Tripods with built-in levels are very handy for crucial shots.
Keep your subject away from the background indoors and out. Indoors, this will prevent ugly background shadows (when using a flash), and outdoors (as well as in), the background generally will be blurred, leading the eye to the main subject.
Never touch a mirror or imaging surface. This may result in permanent (expensive to repair) damage.
