Usability for Nerds/Involve users in the design
Potential users should be involved from the very beginning of the design process. There are two kinds of users that you can involve:
Experienced users. This is the kind of users that have long time experience with using a similar product (a previous version of your product or a competing product). Experienced users often have many suggestions for things that can be improved.
Novice users. This is the kind of persons that might need your product but have never used a similar product. They are useful for asking the silly questions that you haven't thought about. How do I ... ?
Both types of users can be involved in usability tests as soon as you have the first prototype or even just a drawing of the user interface.