Make the most frequent tasks easiest. Avoid using the same site for multiple audiences. As this is not always possible, be aware that each set of users has a different set of tasks and wants to accomplish them without confusion or hesitation. Structural boundaries should prevent users from crossing over to other areas. Be flat, if you can.
Web sites seem to run into trouble when their organization is deeply hierarchical or vertical.
Make options mutually exclusive. Give considerable thought to names for links and menu items. A user should not have to wonder which of two or more options will get the desired results. Make links utterly clear. Research shows that users respond favorably to precisely worded links, even if they are many words long.