Craigslist works great for me.
Here is the basic idea of how I use it. It has changed a little bit in the last year or so, but you can still code your ad in HTML and make it really flashy. Everytime I want to pick up some new business all I have to do is post an ad there and I'll have the following week booked pretty solid. It takes between 3 and 5 days after I post the ad before I start getting solid appointments, but I have known times where I get a call an hour after I post it just because someone was looking at that time.
Don't just post for full details either. Post one for interior cleaning, dealership sticker removal, debadging, wheel cleaning, glass cleaning, etc. Find any area of detailing that you think people might search for and put an ad up. Craigslist is very much loved by Google and you'd be surprised where your business will come from. I picked up a customer from Seattle who was moving down to Salt Lake because he Googled 'salt lake city detailers' and found my craigslist posting. It is an awesome business generator.