Perhaps get in touch with people that collect antique radios. Although old radios used far different electronics (tubes, etc), techniques used to store them might apply considering we're talking about storing an electronic device for 20+ years.
Off hand, I'd say a cool, dry, dark place. You'd want to protect the metal from corroding, too. Perhaps something that absorbs water.. Sometimes new electronics like DVD players and stuff have one of those packs with beads in them (Desiccant?)..