Give the user the chance to ask for forgiveness rather than forcing them to confirm a (destructive) action. Gmail and other web applications are pioneering this one. Rather than asking something like “Are you sure you want to delete this conversation?” they provide a success notification “The conversation has been deleted” with an “undo” button next to it. The insight here is that, although the application must provide a way to immediately abort a destructive action like this, 99% of the time, users actually intended to perform the destructive action. That should be the easy, one-click case, and aborting the destructive action should be the rarer, two-click case.

If the application pesters users with a confirmation dialog for destructive actions, users memorize a multi-step destructive command: click delete, then click OK — and when they accidentally delete the wrong thing, they miss the chance to abort. Many, many applications are guilty of this.