Fogg behavior model is a simple and powerful model to understand what makes products persuasive.
Brief outline: There are three things that needs to be come together in order for a behavior to occur.
Motivation are pleasure or pain, Hope or Fear, Acceptance or Rejection.
How simple it is do the action. How much time it requires, does it cost money, Physical effort, Brain cycles )Requiring you to use lot of brain cycles to perform an action. System 2 from Thinking fast and slow), Social deviance (doing something against social norm), non-routine.
Informing them with a clear guideline to take an action. It could be notification, your app icon, emails etc.
Here is a link to the complete report:Click here