I'd start with looking for Automotive Enthusiast Forums in your area, although it might seem crazy I'd recommend you offer your services for free to begin with on these boards, they most likely have a few businesses or individuals on there already that offer the service so to get their attention you'll have to do things differently. Don't jump in with advertisements as the board might have regulations on who's allowed to sell there, instead bide your time with informative and friendly posts until you've developed a relationship with the members. Then you can start offering your services for free to contributing members, not as a business but as a member. Once you've developed some rapport you can start selling yourself in your signature and through their classifieds section if they allow it.
Another excellent way to get early business and advertising is through groupon, submit applications to sites like groupon and offer your services at an extremely discounted rate. An industry like Auto Detailing will benefit you more if you develop a network of long-term clients that will get their car or cars detailed more than once per year. The true enthusiasts that you reach through your offer will also most likely be members of local automotive groups and communities and if you do a great job will definitely talk you up to their friends and fans.
Also it's a good idea to establish your niche market in the automotive community. The guy that offers detailing services to everyone won't be the first pick for the high end car guys that frequent the boards, attend the race days and meet for coffee on week nights. If you're good at what you do and you only use high end products I'd recommend marketing yourself as an expert that works primarily or only on high ends. This might be counter-intuitive because there aren't that many exotics rolling around but the truth is, even the guys with the souped up Hondas think their cars are worthy of such care and attention. You'll find that the loyalty and word of mouth marketing generated from these communities will out weigh that of offering your services everyone.
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