I haven't done this personally but it is effective as I have fallen victim to it.
Here's what you do...
Post advertisements on Craigslist in your city and/or surround area.. Post used-car ad's with a fair price (usually slightly less then KBB).. Post several pictures of ANY used-car, the crisper the better (prefer toyota/nissan/honda - that's what people want to buy)... In the description of the Ad post "reply with text/e-mail" to see the vehicle, or whatever.
Collect the e-mails/phone numbers and hit them up with some old fashioned e-mail marketing of some featured specials you have going on at your dealership's lot..
Good luck =D