There is so much to be said about this… It's like asking the same for SEO, 10 years ago.
App Store Optimization (ASO) implies leveraging to elements: enhance visibility (high rank in results) and improving conversion (compelling artwork).
Although there are common strategies apps usually implement to achieve this, the tactics will be very different depending on target demographic, competition and nature of the app.
you can find more details (multiple posts) here: http://giacomoballi.com/?s=app+store+optimization