Advance notice: Simply present the rain check to the cashier along with the corresponding product when you reach the register. Since the cashier will need to manually enter the rain check price, make sure to let her know that you have a rain check before you start your transaction.
Group Multiples: If you have several rain checks to redeem in one transaction, group those items together. Ask the cashier if she would like them at the first or at the end. This is also a situation where you might want to let those in line behind you “Pick Another Checkout Lane, Honey!”
Eyeing the Expiration: Each store varies in policy regarding the expiration dates of their rain checks. Some don’t have expiration dates while others range from 30, 60, or 90 days from the issue date. Always redeem your rain checks within the stated time. If the store is still out of the product at the rain check's end date, have them write you an additional one to extend the time you have to get the product.
Keep in mind: You can use a valid coupon on the rain check item (though the majority of stores require that the coupon still have a valid expiration date). Also, most chain stores allow you to redeem a rain check issued at one of their store locations at another nearby one (see your location for specifics).