A very nice example of a MatLab GUI (for plotting direction fields and phase planes) can be found a Dr. Polkings site (Rice University).
Download dfield7.m and pplane7.m and run them in MatLab.
The source to the GUI's constructed in the videos can be found below:

You need to copy both the .m file and the .fig file

M-FileFig-FileComments
calculator.m calculator.fig [simple 4 function calculator]
maple_parser.m maple_parser.fig [enter and evaluate Maple expressions in MatLab]
simple_gui2.m [3D graphcs of sample functions]
simple_plot.m simple_plot.fig [plot a function of a single variable]