Here are websites with traffic I'd recommend you use to promote your next event.
SLO Happenings - download the SLO Happening app in the iBookstore.
San Luis Obispo Vacations events page - I am not sure how you get your event on this page, you may heave to be affiliated with a chamber member.
Both the Tribune and New Times have email blasts that they feature events in.
More General
Facebook If there is a local Meetup group that contains an appropriate audience for your event, I'd recommend reaching out to the organizer. They might post it for you! Any others you'd recommend? Thanks!