I can't answer your question because I have no experience with this, but I'd like to ask you: How did you acquire those first 200 users in the first place? I'd like to publish an application myself sometime in the near-future, but one of my "fears" is that nobody will use it--and I don't know how to acquire users.

By the way, nice job with the application. This is very impressive work for anybody, but especially for a 17 year old. Did you work on this entirely by yourself?