I published 10 apps to Google Play but 1 app was suspended, 1 app was unpublised by myself. I have 1 app that over 500,000 downloads, 2 apps over 100,000 and the rest got 10,000-50,000 downloads. Believe it or not, I spent nothing for advertising! All what I did was present them to a forum.
As I told before, my scenario is very simple: Making an easy app in my language (not English). I started with Horoscope, TV Guide, 101 Romantic Ideas... It's not hard to do but it's unique. At the moment I published it, there're no competitors. In each app I place a "More apps" button to link to all my apps on Google Play. Once again, it worked like a charm.
Another important thing is focusing on the popular device. In my country it's Samsung Galaxy Y because it's cheap so everybody can buy it. If your app looks weird on its small screen, you will be flooded with many bad reviews => your app will be failed.
I used to update my app about each 10 days. Thanks to new "smart system" of Google Play, the users don't need to download the whole app so a 10MB app can be downloaded in only 5-6 seconds. It's important because it increases the rank of your app. I always try to keep my app on top 10 of Top Free category.
That's all my secrets! I used to earn 3000$-4000$/month only from ads. (My record is 5000$). I hope my real story will encourage you to make more good apps.