Save the Date Announcements
Save the date announcements are sent out early, very early in the process to let people know of your big date. It's very informal and is a great way to start building the anticipation. You can get save the date announcements printed to match your invitations or you can make something unusual that hints at your theme. Have fun with these announcements. Don't state your theme plainly, but give a big dose of it in the save the date announcement. All you need to put on the announcement is your name and the date and something as simple as "Hold the Date".
You can also write something clever related to your theme. For a circus theme, send out a bag of peanuts with a small note attached, "Anna's bringing the circus to town for her 15th. Save the date: July 1, 2010." Have fun and your guests will be counting the days down right along with you.