Make it free once a year
@IndieHackers Alex Chernikov (@chernikovalexey) of Gikken made his app Mate free for one day out of the year and pulled in 120,000 downloads and 700 new ratings, which ramped up his App Store rating to 4.67.
He says it's not enough to just change your price to "free" in the App Store. He recommends using subreddits like /r/AppHookup and /r/Apple to promote the free day.
Keep in mind that current users may feel jilted! Alex offered his paying customers his other app for free a few days before the promotion.