Trash your "cookies," small files (stored in your Internet browser) that record the websites you surf—and the transactions you make. Say you go online and purchase a $200 plane ticket from Chicago to Omaha. The next time you visit that site, you could be quoted a higher rate than what's actually available because of your spending history. Avoid this problem by deleting these files each time you log on; detailed instructions can be found under the "Help" option in your browser menu.