Even better than good error messages is a careful design which prevents a problem from occurring in the first place. Either eliminate error-prone conditions or check for them and present users with a confirmation option before they commit to the action.

Progressive disclosure defers advanced or rarely used features to a secondary screen, making applications easier to learn and less error-prone.

Even though it is better if the system can be used without documentation, it may be necessary to provide help and documentation. Any such information should be easy to search, focused on the user's task, list concrete steps to be carried out, and not be too large.

Users spend most of their time on other sites. This means that users prefer your site to work the same way as all the other sites they already know.

If your users have many questions, it's a failure of your primary site design. It becomes not so much customer support, as much as customer complaints.

Even the best designers produce successful products only if their designs solve the right problems. A wonderful interface to the wrong features will fail.

The system should always keep users informed about what is going on, through appropriate feedback within reasonable time.