I've noticed that many guard services firms use craigslist for free advertising. This is pretty much a shotgun approach, and wonder if there are better methods.
I know of one, which is still in public beta. SecurityProfessional has a job section which allows for free posting, and can be reached at http://www.securityprofessional.us/
There's also the Monster.com track, but this has issues because of the associated cost.