For more ASO tutorials and a complete list of strategies to promote your mobile apps, get my mobile app marketing book: http://www.amazon.com/Mobile-App-Marketing-Monetization-thousands-ebook/dp/B00N14RSNY
For more on how to promote a mobile app (Android iPhone) with SEO, ASO and social media sharing: http://www.glowingstart.com
In this tutorial I discuss mobile app marketing, explain how to use ASO (app store optimization), social media marketing, social sharing, and publicity to get exposure for your iPhone or Android app, and get downloads.
Also, try my full course on how to create a business out of your mobile app. The course covers mobile app marketing, monetization, and general business fundamentals pertinent to mobile applications.
First of all, as far as mobile app marketing is concerned, you must rank in mobile app store search by doing proper keyword research, and creating a beautiful landing page for your app. A good landing page can go a long way to helping you increase downloads for your mobile app by converting and convincing people that this will be a great app.
Once you are generating lots of downloads from the mobile app stores, what you want to do is make sure that people are actively sharing your app and inviting friends. To get people to share your app on social media and invite friends, you must create features that will be better after being used together with friends. You want to create selfishly motivated reasons to get people to invite friends. If you simply rely on people to share because they are nice, far fewer people will share your app than they would if there was something for them.
You can also give people rewards for sharing your app on their social media channels like points or extra levels, or premium features. Having more downloads generated by social media sharing will actually help you rank better in app store search (ASO) because one of the search metrics is the rate and acceleration of downloads.