A hot topic at the moment is retaining users. The Application Developers Alliance has a very useful pdf on how to motivate and engage users to boost customer retention. It includes tips on using customer feedback, customer research, onboarding, personalised communications, in-app support, real-time performance monitoring and re-engagement of past users.
The important thing about many of the motivation and engagement techniques is that they need to be designed into the app. There’s no point releasing an app and then thinking how to retain users.