I find that most people looking for a detailer will google for auto detailing in their area or ask a friend. So make sure you have a professional looking website that's listing towards the top of the search results, and list yourself on all the free online directories like yelp, google places, yellowpages.com, etc. Google Adwords was a waste of money for me, but can help while it takes a little time for your website to start listing higher. I've gotten a few reviews on yelp so when someone searches for detailing in my area, that's one of the first things they see, and then they see the 5 star review icon and will click on the link for more info. Whenever you have a happy customer, ask them to please leave you a review on yelp and that helps with the search results. And since google lists it own results on top, get people to leave reviews on your places page too, though Google is being a ***** right now updating the place pages since I think they recently made some changes and messed alot of things up.
Paid advertising has been a waste for me, I get most of my business through free advertising. If you're not on Twitter, get on there and start following everyone in your area, I've gotten a few customers just by doing that.