Miss Castleton's program for the body beautiful is based on the premise that ""what is good for the body internally will also profit you externally,"" so it was with some relief that we discovered that she wasn't recommending cod liver oil shampoos. It will help to have a farm at your disposal for all those lotions, creams, balms, facial masks and hair rinses made of cucumbers and peaches, strawberries, avocados, fresh cream and fertilized eggs. The ablutions vary month-to-month and include some fairly exotic brews: dark tea, sesame oil and lanolin for a ""winter tan""; myrrh, rosemary, orrisroot and Peruvian bark (!) for an ""herbal toothpowder."" Since Miss Castleton, who is beauty editor of Prevention magazine, believes in a holistic approach, there are also menus and recipes -- which resemble the facials -- for lean and lithe figures and a regimen of exercises for still backs, fat thighs and sagging stomachs. The treatments may not turn you into Aphrodite but they probably won't do any harm and the local health food merchant will get rich.