and select All controls under Full Keyboard Access
Now you can use ⇥ (tab) to switch between the dialog boxes.
Note: Unlike in Windows, you need to hit space to trigger a highlighted box that is not the default one! (If this is not natural to you, here's something that helps many to remember tab and space rather than just enter. The space bar is the keyboard version of a one button mouse with the tab moving the mouse's pointer for you.)
Best Answer
Press Ctrl+F7 to enable all controls for Tab key and use it to switch the active button in application dialogs.
This can also be set in System Preferences -> Keyboard -> Shortcuts.