This is a really important topic! Far too often, at the end of an event we turn our attention to the next event without working to continue the momentum we already have.
Just having a plan for using your event content is a key step. You have a lot of channels at your disposal - email, social media, your blog, the event app, influencers, even other events - that you can use to share the event’s content, as well as content created at the event itself (images, session / discussion recaps, etc) that will keep people talking and thinking about your event.
We have a guide on attendee engagement that has a section about post-event that expands upon some of what I was talking about: http://info.hubb.me/creating-engagement-at-your-event-whitepaper
Finally, for general post-event advice (less focused on engagement), check out this infographic: https://cdn2.hubspot.net/hubfs/1925165/Hubb-Post-Event-Checklist-Infographic.pdf
Hope this is helpful! Let me know if you have additional questions.