Two-factor authentication. That means that, in addition to your password, you have some other means of identifying yourself every time you log in, which makes it harder for someone else to impersonate you. Usually it's a combination of something you know (your password) and something you have (such as your cell phone).

A common example works like this: you go to the site, and fill in your username and password. The site automatically sends a text to your cell phone with a code. In order to complete the login, you have to provide the code you just received. That way, in order for somebody else to get into your account, they'd have to know your password, and they'd have to be holding your phone.