User Testing

There’s nothing more illuminating or humbling than a thorough user-test of a UI/site. By putting one’s ideas in front of real people, you get real-world impressions and the more the better.
No matter how much experience you have or how much quantitative data you’ve compiled to say that your ideas will work, it is rarely irreproachable when it comes to people’s varying experience/learning rates.

My advice to any client who wants to be complete and to flush out any bad UI decisions is to budget at least one user-test session into the cycle.