ASAP! PDQ! It's never too early to develop good health habits. Start now to get familiar with your body so that a little mental warning light will go on when you feel a change-it could tip you off if something goes wrong. Part of this Home Bod Study course is, yes, looking at your body in a mirror and (gasp!) touching your breasts so that you know their contours. Once your period has started waving its magic, crampy wand over you, examine yourself once a month, about a week after your period. It doesn't mean you're a high breast-cancer risk, but don't wait for fear to be your motivation later in life. A few facts on breast cancer: One out of eight women in this country will get it; it usually develops around age 50 or 60, although it can occur earlier or later; and the earlier you discover it the better.