For paid apps, one of the most effective ways to increase the downloads is offering time-limited discount. During a certain period, you offer users some discounts when they use some services of your app on the condition that they share your app to their friends and colleges.

Another efficient way is cooperating with influencers. Influencers are individuals with the power to influence the buying and engagement decisions of a certain group of people. Leveraging the power of influencers can be the best way to drive mass amounts of traffic to your app.Developing honest, authentic, and mutually beneficial relationships with influencers will go a long way in helping to promote your app.

Get Featured On The App Store
Pople give much more trust to offical recommendation, if your app get featured on the App Store, the download rates would be much higher. By following the follow tips, there is a great chance your app will be get featured on the App Store :
· Use Latest iOS Features
· Make your App Universal
· Localize your App
· Make a Unique App(Visual Design)
· Get Good Reviews and High-star Ratings
· Update your App Regularly
· Know Apple Well
· Build a Relationship with Apple
· Provide Customer Support

Data shows that about 95% users will take the app reviews as one of the most important reference factors to judge the app, and most people prefer to download an app which has much more positive reviews(https://www.bestreviewapp.com/?from=cr-chris) Then you should get positive reviews to increase the download rates.

Hope those methods are useful to u.