You have written an incredibly well-design app and released it to the world, doing your usual marketing, posting to social media, review sites, getting friends and colleagues and friends of friends to post links to your app.
How do you know which channel is promoting your app effectively? A simple trick is to sign up for Apple's iTunes affiliate program. The key is not in earning the affiliate commission, but the link tracking functionality which affiliate links gives you.
For example, the link for the game Minecraft for iOS is https://itunes.apple.com/us/app/minecraft-pocket-edition/id479516143?mt=8&at=10l4TP&ct=iosdev . Note the key-value for at and ct. The value for at is my affiliate code and the value of ct (campaign token) is what I use to track when the link is clicked. You have to always use the affiliate code that is linked to your account and use a different value of ct (make up something, up to 40 characters long) depending on where you are going to use the link.
So sign up and begin tracking your iOS app store links.
 See what I did there? Now I can track if people click on my link to Minecraft from this newsletter. It works even for apps which I didn't write.