What are your favorite top 15 most important usability principles?

What are your favorite top 15 most important usability principles? Feel free to cite sources and canons to support your answer.

The most important consulting rule that I follow is: "Never solve the problem as stated." Why? because it is invariably the wrong problem, usually being the symptom rather than the cause. Find the root cause and solve that, and then the original problem usually disappears.

I intend to follow that rule right now.