The author of All I Really Need to Know I Learned in Kindergarten left out (at least) one important lesson: Secret knocks are really, really useful. As kids, we'd use secret knocks to keep undesirables (the kids we didn't like) out of our clubs or rooms. As system administrators, we can use secret knocks to protect our systems from crackers.
Last month's "Tech Support" showed you how to monitor filesystem changes with Tripwire, a handy system utility that alerts you to all filesystem changes. Like SNORT and others, Tripwire's just one of many practical security measures that minds your system 24/7.