[stevejabs]

You could add a nag screen to your app that would show if you have a heartbeat API call that returns that the user is using an outdated version.

[deedubaya]

I do this. 90% of my users upgrade within 5 days of an update being released as a result.

"Hey! You're using an outdated version of [APPX]. Click okay to upgrade via the App Store, or Cancel to continue"