This issue is often a lot more complex than it sounds. Tips welcome and appreciated.
RS 2003-08-21: I often make it a rule to design an app in terms of a Tcl API that does not require Tk. All functionality is available over proc calls. In a second step I design a GUI and add visualisation of app objects and triggers - buttons or menu items that call one of the API commands; labels with textvariables that further control the APIs action. Examples: Chess in Tcl, TclMusic.
DKF: This is an instance of the sort of Separation of Responsibilities stuff that you see a lot in (good) professional software.