The easiest way is to do it directly from the plot GUI itself. Click the image of the floppy disk in the upper left corner of the GUI, choose a name and the appropriate format (I prefer .png), and save it to the directory of your choice. The advantage of this is that if you have a 3D plot and you want to rotate it, the saved image saves the rotation as well.
If you want to adjust it later (rotation or other calculations), save it as a .fig file. You can later open it in MATLAB using the openfig function.
