Honestly, you need to post on more than just Craigslist if you want to attract students. There's a lot for lease in Evanston. I would recommend also posting ads in Norris at NU, in the Kellogg building on the first and ground floors, and at Unicorn Cafe, Kafein, Barnes, and Borders. You'd be surprised how many students stop and read those boards. I would also suggest advertising in the Daily Northwestern, both grad and undergrad students read it daily, including the classifieds.
I'm sure the other sources listed are good for those who are not students, but most students, grad and undergrad, don't reading the community papers often, if ever.