Liability insurance is the number one tool you need.
Unless, of course, you have $1,000,000 laying around for when someone sues you because the bike you built for them fell apart at 30mph because you assembled it with no knowledge, a screwdriver, and a hammer....
I don't know about other shops, but when I opened mine, the agent needed to know - among other things - how long I had been working on bikes, and thusly what level of experience I had. ie: Was I *likely* to f@#$ something up and cause someone to die?