By at least logging out of the website you were using.
I was at Fujitsu getting a new battery that day and they have these laptops that you could use to surf the net while waiting for your turn. I saw this on the browser - someone forgot to log off! I was so tempted to just mess with his account but of course, I am a good girl. I changed the url and waited for the next Yahoo!user to screw him up. Hur.
Oh ya, Monkey doesn’t log out his yahoo on my laptop as well. *evil grin*