I've tried multiple forms of marketing. 1. Admob - Got users to the app, but the app did not retain those users and it was quite expensive to even get them to the app. Over $1 per user for me. 2. Tried facebook ads. Just a complete failure for me. 3. I cross promote my apps within my own apps. That does ok. I'm not exactly sure the exact amount of traffic it drives, but it probably accounts for 5% of the user base on the apps i promote. 4. Facebook, Twitter Sharing. This probably accounts for 5 - 10% of new downloads within an app.