UPDATE: i see Amil has posting similar thing, and also the groupon is a great idea, defently do that, what most business owners dont understand is even if your break even on groupon that is still a dream come true because you aquire customers which you can keep and upsell (Home powerwashing, driveway, window cleaning, etc)
Aside from groupon, living social, i wouldent focus on PPC or anything like that right now.
You have expenses. Gas.
You waste time driving back and fourth.
How do you reduce both of these?
Do cars in bulk at one central location. Make power moves. Go to offices and try to show up every friday. Randomly go to a nearby office and tell the people you will do their cars every friday and bill them at the end of the month. It surprising how many of you mobile retailers dont do that. If anyone ever complains about you washing cars in their parking lot do theirs once a week for free, and do a good job. That is more thinking outside of the box than going home to home, you can do bulk price reductions which makes people happier and more prone to purchase. Also employees in an office pressure one another without even knowing it, like hey mike do you want to throw in for the powerbowl? No. Why not its only $1. Ok your right. Same with a car, its only $8 and your car is dirty just do it cheapskate.
Go to commercial businesses with fleets of trucks and offer your services every week. And finally you have car dealers but they are harder considering many people all ready do them. Get your license to do commercial powerwashing, if you hook up with a few of the building owners or supervisors (do their cars real good) doing cars you may get the accounts to power wash the buildings and sidewalks as well.