You could also try getting a promotion from apple in the App Store.
Another thing you should do is incorporate some sort of analytics in your app. Most users wont tell you ANYTHING about your app. I'm running Google Analaytics and I can tell that the average user spends ~7 minutes in my game which lets me know that each session is pretty engaging. That's also 7 minutes that they are getting hit with iAd's if they didn't upgrade. Metrics are key, so add something to track app usage.
Your application is niche so you need to target spaces where people that use your the services that your app integrates with hang out.