So far, i've just been putting my apps on the app store with no promotion. It's going alright, making ~$100/day. For my next app, since i'm putting a bit more effort into it, so i'm going to match that with more effort into promoting it too so i'm trying to figure out where would be most effective. So... long story short: i'll find out soon.

We once tried buying google ads directly on highly targeted keywords. We spent a small amount of money, but it resulted in no sales at all. Other attempts at advertising, via tap joy, etc, also produced almost no positive results.

Using pay-per-install, you can have thousands of people download your app (for thousands of dollars)... but nearly none of them will ever launch it again.

I think they're also the same people who download dozens and dozens of apps, more than they can keep track of.