You should look into UML. This is a standard in not only creating your GUI, but designing your classes, functions, and sub procedures. UML includes many different types of diagrams including: Class diagrams, Component diagrams, Deployement diagrams, Object diagrams, and Package diagrams. I'm sure there are others, but basically UML is a structure and modeling standard that many application developers use. The elements include: actors, business processes, logical components, activities, programming language statments, and database schemas.
Modeling is very important, especially for larger applications where there are many developers and anaylists. It is also nice for smaller applications and for documentation purposes as well.