I think it matters the type of work you are looking for and the size of the city you live in. I think it goes without saying that you need business cards and a website. Other than that, I researched this extensively, and based on my own experiences, research and assumptions...
Big Cities - Yelp, Google Places, referrals, free details for people at businesses/high-traffic streets in neighborhoods
Small Cities - Yellow Pages, Post Cards, Flyers, Referrals, Business to Business (corporate locations, dealerships, etc)
I believe this is true because of competition. In a city like San Diego you are 1 of 100,000 detailers offering the same thing (to consumers it's the same). A post card is 1 of 1000 we get every day and 9999/10000 they go straight to the trash. Same with other forms. Your best bet in a big city is to get out there and make yourself known, life is fast paced in cities, people don't waste their time looking at post cards, flyers or the Yellow Pages. In smaller cities however, I've seen the total opposite.
Mobile Car Washing/Low End Detailing - Yelp, Google Places, free details, post cards, flyers, B2B
Hi-End Paintwork - Referrals, Yelp, Forums (detailing, local, vehicle specific enthusiasts), Personalized business cards/flyers (I have post cards that I leave on people's doors stating that I like their car, I noticed some damage, would love to give them a free inspection...etc)
I believe this to be true based on customer needs. People looking to spend $$$$ rely mostly on other people's opinion, not your "advertised" opinion. Mobile washes/quick details are all the same service, so usually they look for convenience. High-end paintwork though, people are willing to pay for the difference, but only if someone else can make them believe it. I am currently running a special for free details for a few M3 owners ($150 detail that costs me $15 in product?). As a former M3 owner, these mofo's are extremely picky, anal and willing to spend big $ for good work. If I can impress a few M3 owners, I'm hoping it will net instant business. Just (1) detail will pay for it all, and I get to meet/hang out with fellow Bimmer enthusiasts -- it's a win-win for me!
Groupon/Living Social/Google Offers I believe are best for larger businesses. I've seen a few San Diego detailers ruin their reputation and business by signing on with Groupon. Too much business growth and generally cheapskate customers that nitpick and only look for deals.
In summary, I believe the best bang for your buck is Yelp and forums/car clubs/car meets. All are free and only cost you your time and you are marketing directly to people that love their cars.