In the online age, one can't quite say "only you can prevent privacy and security breaches." We rely on so many people and organizations to take care of our data and often they let us down (see Sony, Epsilon and RSA, for example). But the one thing each user can control is whether they are a weak link in the chain protecting their own privacy.
Most recently, Google announced that it had become aware of attempts being made to get into certain users' GMail accounts. What's most interesting is that this was not an attempt to hack into Google, but a relatively sophisticated spear phishing attack focused at the users.
Canadians have also heard about attempts to get access to Canadian government computer systems, again through the users and not directly against the systems.
Users need to be constantly vigilant against these sorts of threats. Where possible, use services that take advantage of SSL encryption and authentication. Be very wary of anything that doesn't seem right and trust your instincts. If you are a GMail user, sign up for the two-factor authentication, which uses your cell phone or other mobile device as an access token that should prevent someone from getting into your account even if they manage to trick you into handing over your password.
For more steps you can take to protect your own privacy, check out http://www.staysafeonline.org/.