Think you need to turn on Private Browsing.
From Safari help:
"Protecting private information on shared computers
When you browse the Internet, Safari stores information about the websites you visit on your computer. That information can include the contents of the webpages you've viewed and any information you've entered at a site, such as usernames, passwords, and credit card numbers. Other people who use your computer may be able to view that information.
If you don't want Safari to store this information, you can turn on private browsing. When private browsing is turned on
Webpages are not added to the history. (You can still use the Back and Forward buttons to return to webpages you've viewed in a window.)
Items are automatically removed from the Downloads window when private browsing is turned off.
Information isn't saved for AutoFill, including names and passwords.
Searches are not added to the pop-up menu in the Google search field.
Cookies are deleted when private browsing is turned off.
To turn on private browsing, choose Safari > Private Browsing.
To turn off private browsing, choose Safari > Private Browsing again, and close any windows you've used to view private information. (Other users can view pages you've visited in any open window by using the Back and Forward buttons.)
Private browsing is always turned off when you open Safari, even if it was on when you last quit Safari."
