Supplemental Consulting Work Advice

I have a full time job. I'm considering taking on additional consulting work (ideally in some sort of fractional manner), but I'm a little suspicious about the companies that have seemed open to such an arrangement. What are some best practices to follow in order to come out of this sort of thing successful and not scammed? I'd imagine "work with people you know" is the first step, but I'm curious if others have any lessons they've learned so I don't have to learn them the hard way. Some friends of a friend (totally different industry than mine) were recently fired for pursuing additional work and it's spurred some paranoia.