Google

If a user opts out, Google will continue gathering and storing information but all data will be anonymous for 18 months.
How to remove your search history:

• Log in to your Google account.
• On the right-hand side of the page, click your email address to pull up the drop-down menu.
• If you want to read more about the policy, click "Privacy." Otherwise, click "Account Settings."
• In the Services section, click "Go to web history."
• Click the "Remove all Web History" button. A prompt will appear to confirm the selection. Click "OK."

You may also want to delete your YouTube viewing and search history. To do this, click YouTube on the toolbar at the top of the Google homepage.

• On the right-hand side, click your user name and select "Video Manager" from the drop-down menu.
• Click "History" on the left, then "Clear all viewing history." Refresh the page, then click "Pause viewing history."
• To clear search history, click "Search History," then the "Clear all search history" button. Refresh the page, and click "Pause search history."