This was shared with me by app developers that hired me to help them promote their iTunes and Android apps: "Admob gets you the most downloads". I am not surprised because Admob (http://www.admob.com) is actually Google.

Why Admob is so good:
They provide detailed instructions:
on how to create, run and pause campaigns
bids, budget, and funding
reporting and tracking

Do not neglect to promote the app in social media: Pinterest, Facebook, Instagram, Reddit, etc. Of course, there are a lot of great social media sites, but I personally saw the best results on the 4 sites I mentioned.

One thing to remember: social media promotions work perfectly and deliver good results but... it takes time. That's why you need to keep at it again, again and again.

There are certain techniques that deliver great results, not just good.

Now go get tons of downloads. Best of luck!
Have questions? Give me a call.