5. Milk social media
Use online communities and social networks to publicise the event. Facebook and CouchSurfing are the obvious places to start. Adjust the privacy settings based on how big and how public you want the event to be. I say inclusiveness is a part of 21st century fun, so the more the merrier.
Make administration easier by opening a gmail account for people to officially sign up through, so they can confirm their attendance, and you can anticipate your turnout more accurately. Post a series of quick, informative messages and send a reminder or two as the event draws near.