Authoring Flash Builder 4 applications can yield some complex syntax filled source code modules. In an effort to alleviate source code clutter and to foster re-usability of code I recommend custom components be employed throughout the development of the application.
Flash Builder 4 also has some nifty features which also lean towards using modularization to ease the maintenance burden of mxml applications.
Once properly introduced to the Flash Builder IDE the components become registered part of the development environment.
To begin make sure there is a components sub directory for you project. Create all components for the application and package them in the components sub directory. Once the components sub directory is populated it may then be exported and made available for import into new project.
The overall approach yields a stream lined mxml application with calls to the components. Also for programming tasks that require a greater level of interactivity these components can be used as action script classes.
Mr. Arch Brooks