If you're looking specifically for online outlets, I've posted rentals before on Kijiji.com also (an ebay owned website) and received good response there. People do still check the local papers though so don't discount those either.
-Postlets: This will post your listing on Trulia, Zillow, Hotpads and a bunch of other sites.
-Domu.com: This is a local site only for Chicagoans. You have to pay to be up there. But I've rented a couple places through this site.
-Facebook: If you have a large network, this could help too.
If you ever need any help, please feel free to reach out to me!
Megan, I'm from San Diego (San Marcos) and miss it about now with a blizzard about to hit us any hour now!