Principal serves up a diet similar to the one recommended by the American Heart Association and most doctors, garnished with some new recipes and some show-biz spice. Using her own body as an excellent illustration, Principal offers the philosophy and practice of her ""Diet for Life,"" plus two weight-loss plans: ""The 30-Day Diet to Lose"" and the seven-day ""Bikini Diet."" Even her most severe program is sensible and balanced, and she warns stridently against fad diets. ""I don't believe in a life of total denial,"" she says, advising most things in moderation--including occasional treats and desserts. She begins by acquainting her readers with basic nutrition, and advising a regimen high in calcium and fiber, low in sodium and sugar. Most of her tips are standard diet-book fare: trimming skin from chicken; drinking a glass of water before every meal. Some are fresher: dipping a fork in dressing, rather than pouring the dressing on the salad: buying a beautiful 8-ounce glass to encourage water consumption. Because of the name--and the look--sure to have wide appeal.