Check out this thirty minute video by Aza Raskin on the value of prototyping to great design.

1. Your first try will be wrong. Budget and design for it.
2. Aim to finish a usable artifact in a day. This helps you focus and scope.
3. You are making a touchable sketch. Do not fill in all the lines.
4. You are iterating your solution as well as your understanding of the problem.
5. Treat your code as throw-away, but be ready to refactor.
6. Borrow liberally.
7. Tell a story with your prototype. It isn’t just a set of feature.

Rapid Prototyping with Aza Raskin from Dan Braghis on Vimeo.