Do some kind of layered design. Something along the lines of MVC as practiced in web development. Put all the logic that loads and stores data in one layer. Put the code that decides what can be done with the data in another layer. The final layer will display the data.

Your display code talks only to the control code, and your data code talks only to control.

By separating all this stuff out, it makes it easy to change UIs. It also makes your code easier to write tests for.

There has been some discussion of how to organize a big gui app here. A bit of supersearching should turn it up.