Becoming involved and contributing to my mobile app's niche on relevant subreddits. I try really hard not to be sleazy and am actively interested in the topic so I try to contribute in earnest while occasionally steering people to my app. I also setup my own subreddit where I post about my development progress during the open beta, this way if people follow it feels more like "opt-in" and gives people a chance to feedback and feel like they're properly involved during the beta period. I also have a standard landing page collecting emails, have tried throwing a few dollars at search ads but didn't really know what I was doing. HN and the other tech related sites don't do much for my specific area.