Better take care of that rocking throne before Big Uncle Earl comes to visit; the last thing you want is an out-of-commission toilet with a house full of company. First, check the closet bolts holding the bowl to the floor. If they're loose, hand-tighten them, then give them an additional quarter-turn with a wrench. If they're already tight, steady the toilet by slipping some plastic plumbers' shims into the gap where the bowl meets the floor. You'll find these useful wedges in plumbing-supply stores and the plumbing aisles at home centers.