Is this even possible, you say?? Yes, if you use some kind of reader (Google Reader, Bloglines, My Yahoo, etc) to view blogs, you can send craigslist searches to go to your reader. That's really helpful if you are looking for something specific. For example, I'm interested in finding a dressform. I go search on craigslist and don't see anything suitable, but I know I'd like to keep an eye out in case a new one shows up. The feed will bring any new listing that meets your criteria to your attention, instead of you having to repeatedly do searches.
So here's how you do it: Go to craigslist and do your search, making it as specific as possible. You don't want more than 2-3 results to come up because otherwise you will be overrun with them in your reader. You can add "-" terms to your search to help narrow things. In the case of the dress form, you might get a lot of results for dresses. You can change your search from "dress form" to "dress form -gown -evening -wedding" to eliminate any posts that also contain the words gown, evening, or wedding. The check boxes on the right side can be useful as well - when I was searching for a dining room table, I checked the "has image" box because I knew that I wouldn't even bother responding to an ad that didn't having a picture.
Once you have your search dialed in, at the bottom of the results screen you click on the RSS button (orange with text "RSS" in white).
That'll take you to a page where you can choose what kind of reader you use. Select your feed reader from the dropdown menu and click "Subscribe Now." You'll be taken to a login page so that it knows who to send the feed to. Once you're finished with that, you can sit back and relax.
I don't recommend using this method if you are searching for extremely hot items - like free furniture on the curb. The feed only updates a few times a day (I'm given to understand that updating more frequently would cause problems - feed readers would decide that the feed is spam and block it.) If you are looking for things that might be gone in 20 minutes or an hour, you aren't likely to get the news in time. In most cases, though, it takes a couple of days to make a transaction on Craiglist, so the feed reader works great.