This one is a no-brainer for mobile apps. Send your users push notifications!
Most people carry their phones with them at all times. That means you get access to their attention throughout the day. Of course, this also means you can easily abuse push notifications. Give your users a compelling reason to enable notifications, and you’ll see retention multiply.
You can use this tactic if you don’t sell apps too. Using a service like PressPadNews, you can turn your site into a mobile app and get it approved on the App Store. Users will get push notifications whenever you publish new content on your site.
If creating an app doesn’t fit your product strategy, web push notifications are a great alternative.