A Clean Duster
You probably use a feather duster or dusting cloth, but chances are it's dirtier than the surfaces you're trying to clean with it. Do yourself a favor and vacuum your duster after every job. That way it will be clean and ready to go next time. You'll pick up more dust instead of just moving it around from place to place. Once you get into the habit, you'll wonder why you never thought of this one before.