Use MeetUp and create a group surrounding your event – this might turn into something very valuable to do on an ongoing basis and create a nice way for you to build a local community.
Publish your event to some of the bigger online events calendars such as Yahoo's Upcoming or Eventful. These sites have geography built in and help promote events that are near users.
Create multiple Facebook pages or twitter accounts just for the event and post relevant information by building local followings through twitter search and Facebook Groups.
Do a series of interviews with participants in the event or to tease out bits of content that will be presented. Record these interviews as postcasts and post them on your event pages, submit to iTunes and offer them to others to run on their sites. Just make sure it’s great content.
Upload transcripts from the interviews or slides you intend to present to sites such as DocStoc, Scribd, and Slideshare.
Include quick videos and photos of before, during and after the event and host on YouTube and Flickr for added exposure.
Submit press releases before, during and after the event to sites such as PR Web and PitchEngine.
Cross post as much information from all of this activity to all of your social networking sites such as LinkedIn, Facebook, and twitter as all allow links to videos, audios and photos.
The short-term impact of working a system like this to promote an event or launch is greater exposure and hopefully greater participation, but the long term impact for future events may be the real payoff. As you get better at this kind of social media routine, you’ll find momentum building through search engine traffic too.
Image credit: Andresmh: http://www.flickr.com/photos/amonroy/