There are a number of software packages out there, though I haven't tried any of them. They're kind of semi-automatic--Craigslist keeps changing/enhancing its security (Captcha or equivalent), so some manual involvement is needed. Still, you can post to multiple locations multiple times a day. Also, the software will rotate the headlines and alter the body text (sometimes by putting in invisible text) to try to avoid Craigslist's detection methods.
Other folks use posting services. You actually can find some advertised on Craigslist. Or try a service like Elance. There are plenty of affordable human posting services, often based overseas. Again, I haven't used any of those. However, don't hire a full-fledged virtual assistant to do it. I tried that, and it was a waste of money--just too expensive.
Also, if you use a human, verify that they're actually posting. (For a while, my VA claimed she was, but wasn't.) No one is going to guarantee that the posting will stay up there for any length of time, but you can at least verify that it's been posted.
Hope that helps.