Be sure to alert the cashier when redeeming a rain check (before the items are rung up) since he or she will need to manually enter the old sale price.
Another great benefit of redeeming rain checks is waiting until a matching coupon comes out or there are more coupons available. This can make a good sale a great one later on! Just make sure that those coupons set aside for the original sale still have valid expiration dates. Most stores do not accept expired coupons.