1) Create the event on your events tab in Facebook.
2) Send a message to all fans of your page to invite their friends.
3) Find other Facebook Pages that would be interested and post your event on their wall. For example, we posted this family friendly event on 365 Things To Do With Kids In Cleveland and Macaroni Kid Cleveland West
4) Use Hootsuite to schedule Twitter messages about the scheduled events. I like to send out 3 at a time to really make an impact. Using Hootsuite can save you time by spending an hour writing messages, but sending them out scattered throughout the day, or even week!
Lastly, make sure you follow up and thank people expressing an interest.