Basically: I built a network and reputation over years as a tech leader in a specific niche (think Java Spring). This was done with articles, speaking at conferences, and building rapport and reputation with peers at these conferences and with co-workers (including non-technical peers). I was the lead developer at a startup for a long time and spent time building the platforms they still use today, which gave me a lot of credibility amongst my peers.
Those relationships were invaluable. The speaking/articles got me into professional networks that then got me referrals. I kept up with former peers enough such that I was able to get both business and employees from them.
I have done zero advertising and have projected $1.5-2 million in revenue this year. Most of that is profit. I'll personally have a net income of over $1 million this year... and at this point I'm spending a good portion of the day playing FPS. ;)