Coming from a fellow owner of a custom paint shop -- I have learned that your reputation is EXTREMELY important. I agree with Taz; Taking care of your customer is one of the most important things which in turn will attribute to your reputation. Clearly, being 100% honest is huge. There have been a couple of times where I "drop the ball", but I run my business with integrity... therefore I do everything it takes to make it up to the customer and then some! Advertising is great but word of mouth is better. If you have a customer that is unsatisfied, I can guarantee that their friends will hear about it. With that being said, If you have a customer that is happy with your work they will tell their friends =). My main way of bringing in business is actually going to car show with my customers. I have had a few customers that I have gotten to know pretty good over the past couple years and they are extremely valuable when it comes to bringing in business.
Once again, I agree with Taz when it comes to materials. Keep a very close eye on what materials make it to your shelves and subsequently to your garbage can. Sometimes materials and even equipment mysteriously grows legs if ya know what I mean. Trust me, I learn that one the hard way 2 years ago. Also, when you start out small you will soon learn that you don't need the best of the best. Get to know your competition and the current state of their business. It seems a bit cut-throat but I only say that because if a business goes under, you can get great deals on things like spray booths, lifts, misc equipment.
Last but not least -- BobM knows what he's talking about. Be sure to get paid for your time. Nobody works for free. Don't be afraid to charge someone what you think your services are worth. It's great if you are, but it's not always necessary to be the cheapest shop on the block. You charge for materials, overhead, your time, or whatever. Maybe throw some deals to friends just to get your work out into the public for people to see.
Sorry for the long post. Just trying to help out. I may still be considered a youngster but I speak from experience.