Call your closest friends and family. Explain that you need extras and you would appreciate their help.
Call in any favors or convince your friends that helping you out will earn them one free favor from you, redeemable at any time.
Create a 'Call for Extras' document. The document should include the time, address, and description of the scene(s). It should also include an overview of the movie and tidbits about cast and crew.
Post this document on sites like MySpace, Facebook, IMDBPro, and Craigslist. Chances are, those who access those sites are likely to already have interest in opportunities like this.
Offer food! Food entices people.
Be prepared to hear the word "no" and/or many of the dumbest excuses any person will ever hear.