The Three Rules of Usability
To make sure you get full value from your usability investment make sure that user testing is part of an iterative design cycle.
Here are three rules, based on the usability standard ISO 13407: Human-Centered Design Processes for Interactive Systems, that will help you to do that:
1 - Early and continual focus on users
You must understand your users and what they want to do. This understanding is arrived at by directly studying their behavior and attitudes, and by studying the nature of the goals you expect them to accomplish.
2 - Empirical measurement
Early in the development process, intended users should actually use simulations and prototypes to carry out real tasks, and their performance and reactions should be observed, recorded and analyzed.
3 - Iterative design
When problems are found in user testing they must be fixed. This means design must be iterative - there must be a cycle of design, test and measure, and redesign, repeated until the usability objectives are met.