Others have given you the details about promoting app on Twitter. You should also promote your app on the app stores, because over 65% users discover new apps by browsing or searching on the app stores. Here are some effective ways to promote app on the app stores:

Boost app ranking
When people searching for new apps, most of them just browsing the top 10 apps, then if your app rank to top 10, you can expect lots of downloads. Follow the following methods to boost your app ranking on the app stores:
· App Store Optimization
· Gather Traffic and Downloads
· Engagement and Retention
· Regularly Update and Upgrade
· Reduce App Size

Maximize Keywords
In addition to incorporating keywords into your description, app stores also ask you to list search terms relevant to the app in the “keywords” section. Include as many keywords as the form will allow to make sure your app is as visible as possible in app stores search engine.Use shorter keywords so you get more key phrase combinations - The more keywords you have, the more possible your app will be get discovered and installed.

Gather app ratings and reviews
As we know, most people prefer to download an app which has much more positive reviews and higher ratings. And data shows that 94% users would like to download those 4-5 rating apps and 75% users will take the app reviews as one of the most important reference factors to judge the app. Besides guide your users leave positive feedback on your app.You can also get app ratings and reviews from third parties(Reviewapp4u is a good choice - http://www.reviewapp4u.com/?from=cr-chris).

Hope the 3 methods are useful to u.