Have access to several social media platforms. As those platforms progress and evolve, what may have worked before may no longer be feasible. One example points to Facebook's reach algorithm. While it claims to reach better qualified recipients, the algorithm still reaches fewer people. While you may consider abandoning one platform for another, keep in mind you can't transplant community. You'll need to start the cultivation process of your new community completely from scratch.
Indirectly advertise. Find ways to mention your organization and/or event in places like constructive blog comments, Facebook reply postings (on other Pages besides your own), and even postings in places like Freecycle. Be sure what you're typing is strongly on-topic, and complies with the web site's rules, regulations, guidelines, and etiquette.
Set editorial guidelines and a schedule. Yes, you can over-post and subsequently turn off people as a result.
Have a virtual press-kit ready.
Have at least one person engage in media training to be sure the message you want to send is consistent and decisive. That one person will use that media training to engage the press and act as a point-of-contact.