Are you near any Military Bases? Try http://www.ahrn.com/
It is sponsored by the military, it is free and it is a resource that is given to members of the military before they are transfered. I've found 2 great tenants using it.
But make sure to also post on Craigslist. Even in a Military area, that is a "go-to" site and typically is not a blocked site.

If you are in need of a place to find tenants, try our website www.HomeJotter.com.

Home Jotter is a completely free service that we provide to students and landlords.

All you need to do is sign up and you have the ability to upload your properties directly to students.

Hope this helps!

www.HomeJotter.com

I assume you're looking for quality clients/tenants. my best advise will be to 1. Analyze the quality keywords related to your apartment such as Luxury apartment in <name of Your city>, and advertise it with Google adwords and craigslist