Hi there,
I take it that the app is already live in the App Store. If you didn't map out and execute a launch strategy, then you might consider doing a "re-launch" of sorts around an update to the app.

There are a number of tactics that can be really effective for boosting organic downloads—two examples: a long-tail keyword strategy for your ASO and press release with extended release through prMac.com.

That being said: I agree with Slaven. I'd be slow to spend money on Facebook, iAd, and other campaigns until you've benchmarked your average revenue per user (ARPU). Otherwise, you run the risk of simply flushing your money because your installs aren't making back the money you spent on them. I just finished writing an app marketing guide for iOS developers, so all this stuff is pretty fresh.

Let me know if you want help mapping out your strategy and tactics.

Cheers,
Austin