Recently we here at TUAW were asked about navigating dialog boxes and applications with the keyboard. It is really easy to use the tab key to select different parts of the window and different buttons in a dialog box, if you know how to enable this behavior.
Open System Preferences > Mouse and Keyboard, and click the "Keyboard Shortcuts" tab. You are presented with two options at the bottom of the window (in the "Full keyboard access" section): Text boxes and lists only or All controls. By default, "Text boxes and lists" should be selected, but if you want to be able to select other buttons in a window with the tab key then you need to select the "All controls" option.
Now when you encounter a dialog box (or window), just press the tab key to switch between the various buttons on the screen. If you would like to go to a previous button, use shift + tab. When you've got the button you want highlighted, press the
For more tips and tricks like these, visit the Mac 101 section on TUAW.