Primary, secondary and other menus
Have you ever wondered, what goes into Primary menu, what should I put in Secondary menu and what is the Navigatio block for?. See the tip below.
Studies on computer graphics suggest that Primary menu which is usually placed on the top of the page in horizontal way should have all the site sections.
Secondary menu of drupal can either be used to expand Primary menu or can be used on the top right hand corner of the page to display information like "About us". "Contact us", "Logout", "Invite a friend" etc. As you will see that these items are not site section or main site content. It is more like extra information to the users, which may not be the main purpose of the site.
Navigation menu in drupal can be used to display "User specific menu items" like "my account". "my subscriptions" etc.
Nice menus contributed module with some minor modifications can be used to display drop-down primary menu.
Tip: Avoid using more than one menu on the left hand side. Try to sum up the site into above three menus