Prototyping. It sounds complicated, but it's really fun. Fail fast and fail often is the motto.
A prototype is nothing more or less than a facsimile or approximation of your idea -- bringing it out of your head and into reality. Any materials can be used, but the most common are paper and cardboard.
For a class on prototyping given by IDEO and Acumen, our team chose to look at a way to promote healthy living at work. Here is a photo of our first prototype:
The items represented were:
The next level of prototyping, if appropriate, is to make a full-size version. This allows testing by real people, leading to refinement (iteration). Note that the version we made for testing has some components that are the same, but others were added, changed, or cut from the first prototype for reasons such as cost, practicality, etc.