If you have an American Express card, log into your account online and scroll down until you see the “Offers For You” tab. Many (but not all) will find a Costco.com offer where you if you spend $115 on your linked AmEx, you’ll get $10 back. (Looks similar to this). If you’d rather spend money in-store, you can buy a $100 Costco Cash card and a $25 Costco Cash card and you will get the $10 back. I just did it and it worked for me.
Get a one-time $15 statement credit by using your enrolled Card to spend a total of $115 or more online at Costco.com by 12/31/2013.
Also look at the other offers, there are some good ones right now including AMC Theaters, Best Buy, Chevron, ExxonMobil, Kohl’s, Land’s End, and more. (The recent Amazon offer appears to be expired.)
You can also try to access this offer via Facebook. You can also use the Twitter hashtag #AmexCostco if you’re synced up.
I was waiting to see if this offer would go wider, but it appears to be limited to those targeted.