Done a little bit with Craigslist, and it's not always the best approach just because you can get a lot of scam, and yes, finding the target market isn't always easy either. The best bet would be to stick to larger markets or markets with high-unemployment rates. NYC, Seattle, San Francisco, LA, Miami, Atlanta, just to name a few. There are plenty of writers in these towns. You should get a hit on your ad.
The content of your ad is probably the single most important aspect of the hiring process. Keep it super specific. Don't be afraid to share contents of the projects you may have them complete. At the minimum, you'll want to include any deadlines and form of payment (PayPal, Check, etc.). Another key element is to include the fact that you will be looking for some long-term employees. You don't want to get stuck with a handful of options that may only be looking for a few pick-up writing opportunities.
Finally, you MUST make sure that you have rules. If someone doesn't follow the simple rules that you give them, they won't follow your simple rules once you hire them. Rules would include: Provide 2 examples of your work in *.doc format. Provide two references. Provide your SkypeID if possible. Provide a résumé, etc. Again, if they can't follow the directions, can 'em.
You might be better off outsourcing through www.textbroker.com at first, and if you like a specific writer that accepted one of your jobs, ask if they would like to have more regular work and outsource directly to them. Odesk is a popular choice as well.
Hope this helps!