I built a free Android mobile news app that was addressing a fairly focused audience of completely indeterminate size (English reading, Barcelona football fans), there were about 3 direct competitors (about 30 related competitors) in the space and they were rated about 3 out of 5 stars with few ratings and fewer comments.
My unique marketing plan was to upload my app and then drive to every mobile phone store in the city I live in and download the app on the demo phones I was checking out in the stores. I then rated (highly) and positively commented on my app with each phone.
If your app is one that a certain focused audience might search for in the Android market or Amazon Appstore then you'll want to be "found" as one of the main pillars of your marketing strategy. My technique raised my search rankings and gave my app the status (high ratings) to encourage and comfort more users into selecting it.