I'm in insurance and investments, and word of mouth is what built my business. I started off part-time in the evenings fifteen years ago.
If you do a good job for people, they'll recommend you to others. It makes them feel like they're offering an inside edge to their friends and family. And then their people start recommending, too.
I heard a story in business school about a painter who would paint the first room of any house free of charge; he would go to a house in a neighborhood and offer free painting. The idea was that the people would see how well he did on the first room and immediately want the rest of their house done, or would remember his name in the future when exploring other painting jobs, or would mention his "first room free with no obligation" policy to their neighbors.
In time, he was able to hire a crew and concentrate on acquiring new business full-time. As I remember it, his approach to customer satisfaction and excellence made him a millionaire in a very competitive and commoditized field in a very short time.