This can be a little tricky as you're not just trying to get traffic but you've actually got to advertise something of value or the ad wont be published! For example, I hope to advertise my main site in the employment section of as many classifieds as i can find. I hope to word the ads something to the effect of:

Learn how to Make money online

No CDs DVDs Videos or Joining Fees - Its Totally Free!

Just good ideas, suggestions and advice from someone doing it!

See although i am just driving traffic to my site i am actually providing "A Free Training Course in Online Entrepreneurship" kind of like selling a free e-book, it is free, but they have to visit my site, if they find it useful, hopefully they'll be back!

But this is not the only kind of site you could use classifieds for, for my aShops where i sell products i would place ads in the online classifieds in the specific section where i know something good has just come out and drive people to the site with that kind of offer. Its just a click away, why wouldn't they have a look, especially if the offer sounds intrigueing!!