Send out a press release and some really good agility action photos to all the regional papers. Most papers will publish a photo as well as a blurb in their calendar or events section encouraging people to check out the event. Sometimes the paper will do a follow up and send someone out for the actual event. Also send out a notice to be printed in any dog publication that lists current events in your area, like the Match Show Bulletin or Canine Companion. Send out extra premiums to people who have asked to be on your mailing list and people who have taken classes with your club.
Radio stations can also be very helpful.
If you can arrange it a few months ahead of time, you can sometimes talk the station into doing a live broadcast from your trial. Most radio stations provide this as a public service, but you have to send them the information well in advance. Also post flyers at all the pet and farm supply stores and grooming establishments. When you can, try to get newspaper coverage. That's a little tougher.