At Social Tables - seating charts for events made easy - we picked a very early group of people to market to who could benefit from our app: brides. Today, we have over 1,000 users (we've been quiet for the past 1 month as we contemplate our future). Here's what worked:

1) Guerrilla Marketing - We found real life events that catered toward brides. For example, the Running of the Brides is an annual wedding dress sale hosted by Filene's Basement, so we showed up really early in the morning to talk to brides waiting in line.

Here's the write-up: http://blog.socialtables.com/post/6147037160/social-tables-f...

2) Social Media Targeting - We monitored specific keywords "wedding & seating chart" and "just got engaged" and replied to each of those users telling them there was an easier way to create seating charts. We also participated in Twitter chats (here's a list of chats: https://docs.google.com/spreadsheet/ccc?key=0AhisaMy5TGiwcnV...) and people got intrigued. Finally, we actively tweeted using conference hashtags during conferences that had our target audience.

I hope these two out of the box approaches help!