First, I followed the advice of people lower in the thread and got embedded in two niche communities (private schools + small business owners in my local area). Second, given that in the past I wasn't a talented networker (that's changing, albeit over years, not weeks) I partnered with two other developers who were skilled networkers and observed how they generated leads and closed sales. The combo of [exposure to potential clientele (1) + knowledge of how to talk to and close those clientele] has been effective.
Another note: all my biggest clients have come through relationships - personal friendships and past customers of software I've created. It's a patient game, for sure, but after a couple years of slowly but steadily completing projects and doing the right things, I'm booked out 6-8 months and have a sustainable pace of work.