My approach is to break the project into small milestones — especially in the beginning — and align the incentives.

Put bluntly, I design it so I can't lose, and neither can my client. I design it so even in an adversarial, zero trust environment, the relationship and arrangement still makes sense.

In practice this looks like, I do a bit of work up front, ship a demo within the first week, if they're happy, they pay, it becomes theirs, and we continue working together.

I also choose projects I actually like, and that align with my goals, so even if I get hosed, I had fun and learned cool things. (But keeping milestones small minimizes the cost of getting hosed, for both parties :)