I can tell you how we do it (small company, 2 engineers + 1 part-time accountant): Word of mouth.
Do everything to the best of your ability - and try to improve that ability with time.
Recognize that it is not a perfect world, and you won't be, either. To err is human - it happens to everyone. How you handle it makes all the difference.
When someone asks you something, tell it like it is. Don't sugar-coat it. If something is going to take 3 weeks to get done, tell them it's going to take 3 weeks ... not 2. But then get it done in the 3 weeks that you said it would take. Say what you are going to do, then do what you said you were going to do, when you said you would do it.
It will take a while.
Our company has not paid one red cent in advertising, ever. We have never cold-called, we have never done anything that all the marketing wizards and expensive consultants say you are supposed to do in order to grow your company and to market yourself. Nothing. But right now, the biggest problem is that the answer I have to the inevitable "I have something for you to look at, how soon can you get here" (which happens on a daily basis ...), is not a very good one, because it's weeks away. And I tell it like it is. "I won't be able to get to this for about 3 weeks, we should pick a day now to make sure it's on the schedule".
I can tell you that it was not always like this ... in the beginning, I was working about one or two days a week. There are times when I wish it was still like that ...