What's your method for settling interaction design disputes?

In speaking with a few project managers recently while consulting the topic of what to do when there are disputes over UI designs arose. This seems to be a recurring theme with more and more people I speak with these days. It's the old too many chiefs not enough indians problem.

Surely extensive user tests and hard evidence can silence a lot of this talk, but what if you only have time to do "quick and dirty" UI tests? How have others handled this in the past? What if you don't have access to actual users of the product? What if the sample size for users is very low (1-2 people)

I'm interested to hear other people's experiences in this subject...