There is a product I like manufactured by Rico called the "Reedvitalizer." Essentially it's a ziploc sort of pouch with a humidity controlling packet inside. Store reeds in that before or after they've been played and it keeps them just moist enough to extend life but not mold. After something like six months you may need replacement packets, but it varies.
The alternative is to just store them in a plastic bag. It'll save you the cash and work just as well - the reeds just have a greater chance of molding after awhile. No big deal, just dip 'em in some peroxide. Give it a shot!