Whenever I start a new software project, one of the first things I insist on is that the design include a command line tool with the same functionality as the GUI. This usually begets a sensible API between the business logic and the shiny parts, which ends up allowing for a great deal of flexibility for the GUI.

I’ve been burned too often by the GUI first, top-down approach, where the software architecture evolves from the GUI. V1 gets shipped, then the UX designer gets bored and v2 has to have a totally different look and feel and you’re screwed because your software design is permanently tied to the original UI.