Where can I find high paying writing freelance jobs?

I usually refrain from promoting my Dr. Freelance entries here, but I'm making an exception for a guest poster, All Freelance Writing's Jennifer Mattern: "Where can I find high paying freelance writing jobs?"

Her sage advice applies not only to freelancers who focus on writing and editing, but any creative field, as well as entrepreneurs of any stripe. If you hope to find gigs that pay well, you need to go beyond the publicly available listings of what's available out there — and Jenn's provided some excellent, practical thoughts on exactly how to do that.