Some iPad apps such as Apple’s iWork suite, include an Undo button in the program’s toolbar that you can tap to reverse your last action.  The availability of an Undo button depends on the app, however.

For standard iPad apps such as Mail and Notes, when you make a mistake like accidentally deleting needed text in a Mail message for example, hold the iPad firmly with both hands and carefully give it a quick shake.  A dialog box should appear on the screen with the option to undo your last typing action.  The shake-to-undo technique also works on the iPhone and iPod Touch.

This tip comes from J.D. Biersdorfer writing in the New York Times.