Via Doctor of Credit, Discover is currently offering a potentially lucrative promotion: for the 12 complete billing cycles after registering, they will double all the cash back you earn with your Discover it card. The cash back will be deposited in the 13th billing cycle after registration.
There are a few nuances here: if you're an existing cardholder, you have to call to register for the promotion — registration isn't available online. However, by all accounts Discover's phone representatives are happy to register anyone who calls; my phone call took about 4 minutes, beginning to end.
Additionally, registration doesn't take effect immediately: since the promotion is based on 12 complete billing cycles, you only start earning double cash back on the first day of your next billing cycle after registration. In my case, that's June 3, but if you have a statement closing date later in the month you might not be able to maximize the rest of this quarter's "Restaurants and Movies" rotating 5% cash back category (Doctor of Credit helpfully suggests asking Discover to change your statement closing date in this case).
Doubled cash back earning — 10% cash back in bonus categories — might make a few otherwise-boring deals worth pursuing. In the third quarter of 2015, for instance, department stores are a bonus category. 5% cash back on $200 Visa gift cards from Sears is a good deal, but 10% cash back on Visa gift cards is a great deal (netting $13.75 per card). You can also do whatever it is people do with Sears gift cards.
If gas stations return in the first quarter of 2016, PIN-enabled Visa prepaid cards will be a great option, and hopefully the always-popular restaurant bonus category will return next year as well.
One thing that's unclear is whether cash back earned through Discover's lucrative shopping portal will also be doubled. I believe it likely will be, but regardless of what Discover's phone representatives say, there's no way to know for sure until 13 months from now!