www.mycomputerguy.net/scottwax is my detailing website.
I've been in business for almost 8 years now. All my clients are private owners-no used car lots for me.
Make sure you get your DBA (doing business as) so you can open a bank account-depositing a lot of checks in your name sends up a red flag and they will not deposit checks with a business name into a personal account. A DBA in Texas costs about $10 and is good for 10 years.
Check into tax laws for your state and federal taxes as well. You don't need that kind of trouble.
Find a dependable supplier. Nothing worse than running low on product and not being able to get it.
Make sure you are listed in the Yellow Pages-a small ad is pretty cheap initially (about $40 a month in the Dallas area), but it will go up each year-basically, they give you a break when your business is new. Once you are established, you should be able to cut back to just a listing. I also used flyers when I started. I would put out about 500 a day in nice neighborhoods.
You also might want to offer some sort of maintenance program. To get steady business, I would wash/vac/dress the tires every two weeks, plus take care of any interior problems and wax a portion of the exterior (after an initial detail, of course) for $35-40 for cars and $45-60 for SUVs. What you are offering the customer is a vehicle that will always look detailed, and wax protection that will never be more than 6-8 weeks old. I still have a few people who do that, but since I have so many who get their cars cleaned each week, I have stopped offering it, mostly because I just don't have time.
If you do good work, you will be amazed at the amount of repeat business you will get. 94% of my business is repeat or referral, so I no longer advertise.
There is a lot more, but my fingers are tired from typing and working-I washed an RSX, CL Type-S, Ford F-150, Lincoln Navigator and a Cadillac Escalade today.