In some of the projects on this list, the client saw a working version of the system before ordering anything and before paying anything at all.
Not screenshots, not slides - a panel you could open in a browser, click around in yourself, and check whether it actually looked right.
We do this on smaller projects, where building a demo is a cost we can take on ourselves. On a large rollout it would be too much work to do blind - there we start with a free workshop, a written-down scope, and a screen map instead.
Trust has to run both ways, and at the start it is us who has more to prove. When the scale of the project allows it, we would rather go first and show something working than try to convince anyone with promises.