Mr. Henderson PresentsLecil Henderson doesn’t take no for an answer. Last year alone, he persuaded retailers to host 90 trunk shows for his Italian-made flexible jewels.His most recent convert is Gold Casters in Bloomington, Ind., a store that had recently picked up Henderson’s new silver and enamel line (starting retail: $450). Initially, owner Brad Lawrence balked at the designer’s suggestion: a midweek show in the blazing July heat. But it was tough to refuse Henderson’s offer: no outlay of cash. “Lecil expected nothing as far as expenses for travel, hotels, and meals,” says Lawrence.Then there were gift boxes for Gold Casters’ top clients: sturdy white cardboard boxes containing a Henderson Collection catalog, a handwritten note from the designer, and a tube of sugar cookies from a Charlotte, N.C., bakery (Henderson’s hometown). “The gift box was powerful,” says L