Alexandre Grendene never runs out of new ways to sell plastic sandals.Sales at his company, Grendene Group, will hit $1 billion for the first time this year, a 21% increase over 1995. Mr. Grendene's secret lies in making heavily ad vertised fashion items out of plastic sandals priced at $5 to $20 a pair.His main brand, called Rider, sells 50 million pairs a year in 70 countries and accounts for one-third of the company's sales, or about $350 million. Mr. Gren dene's latest innovation was to extend the unisex Rider brand in July with a special version for women, called Grendha, and another for children, with the first strap at the back to stay on tiny feet. Between July and November, 1 m illion pairs of Grendha sandals were sold.Ads by one of Brazil's most creative agencies, W/Brasil, São Paulo, featured Dean Cain and Teri Hatcher, the U.S. actors who star in the "Superman" TV series. The spots portray Grendha as the sandal of "superwomen," who are irresistable to "supermen.""We invest in the future and need to give value to our product," said Mr. Grendene, 46, who is co-president of the Grendene Group along with his twin brother, Pe dro. The family company is based in a small town in southern Brazil, and Pedro Grendene runs the group's rubber business, Vulcabras.Alexandre Grendene instilled greater value in the product by modeling Rider, now Brazil's most popu lar shoe, on sneakers, even adding features similar to the Air systems made popular by marketers like Nike to make Rider sandals lighter and more comfortable than rival products."The secret?" he says immodestly when asked of his success. "We are being very competent."Sophisticated commercials costing up to $500,000 each by longtime agency W/Brasil link Rider sandals to the best moments of one's life, using the theme "Give your feet a holiday." Ads typi cally feature famous sandal-wearing Brazilian soccer players and vibrant soundtracks of Brazilian music.Rider's latest TV commercial tells the story of a group of teen-agers who create a special code, called Riderometro (also the t itle of the commercial), based on the sandals. Different colors have different meanings: Red Riders placed outside an apartment door, for instance, indicate that one of the group is entertaining a gorgeous young woman inside.Next y ear, Mr. Grendene hopes to boost Rider's sales by as much as 60% when he takes the brand into Asia for the first time.