RUTH ANN McCLAIN, a flutist from Memphis, used to suffer from debilitating onstage jitters. "My hands were so cold and wet, I thought I'd drop my flute," Ms. McClain said recently, remembering a performance at the National Flute Convention in the late 1980's. Her heart thumped loudly in her chest, she added; her mind would not focus, and her head felt as if it were on fire. She tried to hide her nervousness, but her quivering lips kept her from performing with sensitivity and nuance. However much she tried to relax before a concert, the nerves always stayed with her....