Use a two-factor auth plugin like https://wordpress.org/extend/plugins/google-authenticator/ It works with the Google Authenticator app. Duo Security is also good: http://wordpress.org/extend/plugins/duo-wordpress/ The WordPress.com team have already announced two-factor auth support for wp.com blogs, and are working on an official solution for wp.org sites: http://macmanx.com/2013/04/12/two-step-authentication-on-wo…
Two-facor auth just adds to complexity, and that is a bad thing when it comes to secutiry. You want to be able to easily understand that a system is secure. The more complex a system is, the larger the likelyhood of a surprise "whoops, I overlooked that" somewhere down the road.