Oval Charm
After beating the Elite 4, return to the Game Freak office in Heahea City. Talk to Game Freak Morimoto, and he and Game Freak Iwao will challenge you to a double battle. If you win, you'll be awarded the Oval Charm, which increases the chances of eggs being found at the Nursery. You can also battle Morimoto and Iwao once a day afterwards.