Prototyping is an indispensable tool for design. You have an idea of what you want your product to do, who the user will be, and what the product might look like, but you need feedback. Feedback from users, designers, and other stakeholders, will tell you if you’re on the right track. Hand a user a prototype and ask him to perform a task; you’ll quickly learn how many of your assumptions were right.
The key to an effective and efficient design process is to prototype earlier and often; the earlier you produce and test prototypes, the easier it will be to implement changes in design, and, ultimately, you will get a better product.
Gamasutra has an article about prototyping an app for Nintendo’s Wii U. The developer wanted to see the interface in the real world and be able to touch the device, putting himself into the user’s shoes.
His solution involved bits of cardboard and glue.
This kind of paper prototyping is fast, cheap, and very powerful. Within minutes you have something you can put in your user’s hands (or just in front of him) that can be manipulated, modified, or torn up without much grief.