For my dvd's and those I rent with scratches, I use liquid car wax. Put a nice coat on, let it dry completely to a white powdery finish and then wipe off with a smooth, lint free cloth (I use cloth baby diapers). Works like a charm, and I love returning them better than I got 'em.
Edit: forgot to mention, its important that you wipe from the center to the outside. ALWAYS from the center ring to the outside edge in a straight, smooth motion.