App store optimization (ASO), or improving the visibility of the app within an app store to help it get discovered by relevant users, is the most important factor when it comes to marketing apps. It all starts with having relevant keywords in the app’s title and description. Do thorough research before you get started!
You should also spend time making sure your app is in the right category based on relevancy and level of competition. Other ASO influencers include having an eye-catching logo and in-app screenshots. Finally, getting people to leave reviews on your app is key. Create prompts and incentives to encourage your users to write a review.
Advertising on social media is one of the most effective ways to boost your installs. Facebook’s advertising platform is a great way to reach a relevant audience likely to engage with your app. Twitter’s promoted tweet feature is another worthwhile avenue to consider.
Viral mechanics within the app provide options for users to share with their own networks without leaving the app itself. For example, if it's a game, make sure that they can seamlessly share their score with Facebook or Twitter.
Leverage other people's audience by connecting with influential people or outlets who could drive users to download the app. For example, getting mentioned in Mashable or The Huffington Post would add credibility to your brand and reach a new audience. You could also consider paid advertising on sites or blogs with highly relevant content for your app.
Good luck!