I must confess that I am not a Jakob Nielsen accolyte, but I respect his work and, darn it, most of the  things he says are right on point (and the rest isn't far off!).  This one was a keeper to stoke my usability thinking ...

Parallel & Iterative Design + Competitive Testing = High Usability (Jakob Nielsen's Alertbox)

You can achieve a high-quality user interface by combining 3 design process models:

1. Competitive testing
2. Parallel design
3. Iterative design