Principles of Graphical User Interface Design
The Illusion of Manipulable Objects--objects must be created that are representative of the task to be completed and appropriate for the intended application. They must look selectable and it must be self-evident what to do with them.
Visual Order and Viewer Focus--attention must be drawn to functions and tasks at appropriate times. Feedback must also be provided to the user at all times. This can be accomplished through a variety of techniques including use of color, animation, pointer changing shape etc.