These days, I don't think there is a guarantee of anything, even if you have a few thousand downloads. Even with the initial downloads, my app languished on the store until I introduced iPad support, added more song packs, and improved the keywords.

I think the biggest thing was the simple and catchy name, and a decent looking icon. This will help you get the random users who search for "piano" on the App Store. Of course, being a free app helps. People will download pretty much anything that's free. :-)