Start with observed reality, not assumed reality.
Every feature should be traceable to a real observation of a real person doing the work. Not "users probably want X." Not "wouldn't it be cool if." If we can't name the worker whose conversation motivated a feature, the feature doesn't ship.
How to hold us to it
Ask us, about any feature: "who told you this was a problem?" We should be able to name three people. If we can't, the feature shouldn't be there.