Figure out something you have a use for, and program than. Here is an idea for a program I could use and may write myself. I offer this only as an example, but you're welcome to write it for me if you like!
Site allows you to enter a list of categories and search terms.
Whenever you visit the page, the system searches a specified craigslist website for each of the search terms you listed within the specified category.
The system then consolidates the results (so items matching more than one search show up only once) and formats them for display.
The display includes a link back to the original craigslist website.
Example: I specify that I want to search the furniture section of the SF Bay Area craigslist site for "sofa", "console table", and "Ethan Allen". Whenever I visit my search page, I see a current listing of all ads matching my search criteria.
Extra Credit: Extend the idea to other sites (ebay, for instance), still presenting a consolidated, current listing.