I did it twice, started small just on my own and found it almost impossible to make the jump into hiring a tech. Both times I was soon all too occupied with administrative things and wound up working way late to get the revenue producing work out. Then you have to make the leap to hire a good tech so you now both need to make a living. I tried hiring a low end helper but none of my spoiled customers want him touching their car and I tried hiring a girl to answer the phones but none of my spoiled customers wanted to talk to her. Then I started doing favors for the divorced waitress crowd by basically charging them what they could afford and I turned into the broke single mom welfare shop by word of mouth.
So, I would suggest to specialize and be the best at what you do, treat customers with indifference because you are the best and they need you. Be honest and don't blow smoke to cover stupid mistakes that we all make, bite the bullit when that happens and never ever make your customer pay for your education (or lack there of).
I also used to think I needed to talk everything to death, explaining everything and the processes involved. Do not get into that rut unless they are on the clock. The customer doesn't need to have an auto-tech 101 class to get a repair done, let them educate themselves on their own time. Don't even offer half baked work as a possible option, if you can't bring the whole system back to factory specs then you can't stand behind it and they need to go down the road. Be very careful with aftermatket parts and try to use OEM as much as possible, and remember reman=junk and come backs.
And on comebacks, drop whatever it is that you are doing and address a comeback IMMEDIATLY. You already have that customers money and they deserve immediate attention. Send a wrecker and pay for it if you have to. I cultivate my best customers by how I handled comebacks. Admit you are human and don't try to cover your mistakes with BS.
Hey, I could write a book here so I better stop now. Oh, one other thing, there isn't any money under the hood in a car, don't get into drivability stuff, doesn't pay to be smart here. All the money is under the car, that is why there are so many whore shops raking the cream off the shocks/struts/brake work.