Until I started couponing, I don't think I'd ever even heard of a rain check...much less gotten one. Most stores will give you a rain check for an item that is in their sale paper that week but is sold out when you shop. What a rain check does is it extends the sale price of the item for a period of time so that you can purchase the item(s) at that same cost when the store gets more in stock. Most rain checks are valid for 30 days from the date of the rain check. These come in VERY handy when there's a really good deal with coupons but you seem to be the last person to get to the store.
I don't use rain checks all the time, but if I'm going to get something very cheap or free, and I know the deal won't come around again any time soon, I get a rain check. This also allows me time to obtain more coupons for the item (or to just get some if I don't already have them). It's less stressful, too, as you can redeem the rain check at any time during the month after you get the rain check...just be sure the coupons don't expire first. You can also speak to a manager and ask them nicely if they will order you some extra of the item so that you can use all of your coupons at once with your rain check. The two times I've done this, the manager has been more than happy to do so.
I recently got a rain check on the B1 at $1.99, get 2 free on the Scotch tape at Walgreens. I got the rain check on May 20 and the tape didn't come in until June 30!!! The day the coupon expired. Thankfully, I'd not only ordered the quantity I needed (so that I wouldn't clear the shelves when it came in) but they also extended the date of the rain check.
To get a rain check, you usually just have to ask at the customer service desk. Each store has its own form for rain checks. Just make sure that the item you're requesting a rain check for is listed in the store's ad for the week. Then all you have to do is hold on to your coupons and your rain check and wait for the store to get more in stock.