Mailing lists.

I subscribe to the Debian security advisory mailing list, and when new updates are released I ensure all that are appropriate are updated upon my systems.

I also use an RSS to XMPP gateway so that that the RSS feeds of Debian, Ubuntu, *BSD, etc are posted to an internal chatroom. Generally automated updates are applied every day, but sometimes it is nice to do them immediately.

(I also pay attention to oss-security, full-disclosure, etc, etc, but these tend to be less useful.)