While there has been much successful work in developing rules to guide the design and implementation of interfaces for desktop machines and their applications, the design of mobile device interfaces is still relatively unexplored and unproven. This paper discusses the characteristics and limitations of current mobile device interfaces. Using existing interface guidelines as a starting point, a set of practical design guidelines for mobile device interfaces is proposed. An experimental interface was created that ran on a Pocket PC 2002 mobile computer and used a simple events registration style interface to enter data of basketball’s competition. The buttons of the registration system were changed in size between 12x12, 16x16 and 20x20 pixels and used a sound. Results showed that sounds significantly improved usability for both standard and small buttons.