Find a doctor these days and head down to the corner drugstore with a few prescriptions in hand, and you can walk out with a sack of goodies that promise to make life better. This is the new frontier of drug development : quality-of-life pharmacology. The products on the market like Prozac for positive thinking or a cream called Retin-A to make wrinkles fade are merely the first wave. In the laboratories of the U.S. drug and biotechnology companies, a dazzling array of new treatments is under development.