MOST EFFECTIVE FASHION STRATEGY of all is fabric that behaves. The supple crepe sheath - for instance, topped and bowed with satin. It drapes and conforms exactly as Rudolf, the designer, planned it; weight, surface, texture - perfect! Onondoga crepe Cri-Cri of Enka Rayon and acetate. About $110.00 at Tailored Woman, New York; Bonwit Teller, Philadelphia; Montaldo - all stores; J.W. Robinson, Los Angeles. Modeled by Dovima.