Even better than good error messages is a careful design which prevents a problem from occurring in the first place.

Rather than allowing the user to ship a package to an invalid address, the system is cautioning the user that this may be an invalid address.

And to avoid having the user enter another invalid address, several valid ones are offered in the "We Suggest:" list box.

Make objects, actions, and options visible. The user should not have to remember information from one part of the dialogue to another. Instructions for use of the system should be visible or easily retrievable whenever appropriate.

The Address Validation page does not make the user remember what address they entered.  Instead it displays it as "You entered:".  This allows the user to recognize what they entered rather than having to recall it.   By recognizing the address the user will likely quickly see any mistake.

The instructions for this page are clear.  The error message states the situation and the sentence "What would you like to do?" asks the user to choose the next action.  The choices of action are clearly communicated in the button titles.

January 2001 UPS Address Validation page.