1. SEM - Google AdWords mostly. I spent a fair time on this, partially because my day job at the time was in the SEM field
2. Organic - I got a great domain that contained my primary keywords, got a landing page built, and it ended up driving a lot of organic traffic. I also set up a blog on the main site. The blog was good for traffic, but not that great for conversions.
3. Referrals - my app is mostly used by people that hire domestic help, so I tried talking to the agencies that help people find that help. Hard to measure that one.
4. Provide awesome customer service - I have been told by several of my customers that they have sent their friends.
5. Free trial - The app gives a 30 day free trial. Many customers have thanked me for that.