When I first started my mobile detailing business, I relied mostly on flyers. I would put them out at DFW and Dallas Love Field, until I was told that it wasn't allowed. After that, I went out to the rich neighborhoods and put them on doors. Took longer that parking lots, but no worry of security chasing me off. Any time I didn't have an appointment, I was putting out flyers. If I had a car in the morning, but none in the afternoon, I would stay in the area I was working in and hit as many houses as possible.
I got Mike Lalor of the Dallas Stars business doing this (he has since retired and moved to Wisconsin). I was walking up to the door to put a flyer on it, and his wife was opening the door to get the mail and I almost ran into her. I handed her a flyer and she made an appointment on the spot for the next day. I did his cars and Craig Ludwig's cars and Harley until Mike retired and Craig left to coach a minor league hockey team in Minnesota. I have several other long term customers due to either flyers or business cards.
I also have a sign on the back of my car with my business name and number. One person saw it and told his boss. I have been doing their cars for over 8 years now-between the big boss, his partner and their CFO, I get about $10,000 in business each year, because they get their cars washed every week and waxed at least once a month--high end stuff, too. A Bentley Continental, Lamborghini Diablo, Mercedes CL500, S600 and ML430, BMW 740iL, Cadillac Escalade, Lincoln Navigator and a full sized tour bus. Definitely the type of clientel you like to keep happy.
If you are not listed in the yellow pages, do that ASAP. I started off with a small 3 line insert ad and used that for a couple of years, they just went to a listing. Only one other mobile detailer in the Arlington Yellow Pages, so since my business name denotes that I am mobile, I believe an actual ad is unecessary. No drop off in business since I went just to a listing either.
I no longer advertise--no flyers at all. I may drop a business card here and there, but I rely on word of mouth and repeat business and since 94% of my business is repeat or referral, it is nice to have been able to ax the advertising budget. I occasionally give price breaks to regulars who refer business to me, but that is about it.
Your absolute best way to improve you business is to be the best detailer possible. The fact that I am mobile gives my customers the convience of on-site detailing-much easier to have me come out and clean their 5 cars at their house that have them drive them one by one to a shop. You want to find an extra selling point your competition doesn't have-high end waxes, QD included in washes, stuff like that, in addition to being better than the competition.
Another suggestion--do not undercut your competition's prices. Being the lowest price will suggest to potential customers that you use low quality products or cut corners. Try to be at least a bit above average.