Ask your cashier or customer service person for a raincheck! Here's why.
If I am buying Yoplait Yo-Plus for example, on sale for $1.25 each. I have a small stash of awesome $1 off coupons, making the yogurt 25c each. They are in-stock of some flavors, but our favorite vanilla is out of stock. I choose the in-stock flavors that I like (using my coupons), and ask for a raincheck on the vanilla. Half the time, the raincheck will not specify the flavor, but if it does, that's okay, too.
Now I go home, wait, and look for more coupons. Guess what! There are new printables out the next week. I receive some more through trades. My mom or MIL sends me some more! I can trade for more on my trading forums. Now I have cheap yogurt in the short-term from the yogurt I did buy, and I can come back in a couple of weeks and buy more yogurt for 25c per pack!
Another tip: If you have to specify a quantity on your raincheck, ask for way more than you think you think you will need. You are never obligated to buy that many. Asking for a larger quantity gives you the flexibility to buy more if you find a big quantity of coupons.
Happy dance indeed!
(BTW, I'm buying yogurt this week with my Publix raincheck. They were out of vanilla.)