Add value to your app. Be it content or w/e. And force users to install the app to in order to get hold of that value. Just tickle their curiosity or w/e might bring them to your app and then lock the juicy specifics.
Build user engagement.
Remind users that already have installed your app via posts and notifications. Think very well what you are going to post and what notifications are you going to push. Think about every angle.
As you didn't say what exactly is the app about you are going to get only meta answers.