What's your business model? Paid app, app with ads, OR free with in app purchases? The method I think is best is free with in app purchases. However whatever method you use, my suggestion for promotion would be social media. Do it yourself, try to get others to do it for you. IF you have a paid app, you can give away copies to social media influences, that is folks with lots of follows, or views on YouTube etc. For yourself you can make your own YouTube videos. Also you can run contests via your social media, make a viral video, have folks retweet in order to enter, etc.