The opt-out cookie is not a google.com cookie, it is a doubleclick.net cookie. I allow Google cookies on my systems but not doubleclick.net cookies. So the Google preferred opt-out method wouldn't work unless I made an exception.
I could use the Firefox add-on mentioned, but I have a better way. I use the Firefox add-on "Adblock Plus" along with a
subscribed filter lists that blocks all Google ads along with Google analytics tracking cookies at thousands of sites. I use Easylist and EasyPrivacy at »
easylist.adblockplus.org/Of course Adblock Plus blocks more than Google ads. It blocks ads from all vendors on just about every site I ever visited.