There are a couple of metaphors that tend to guide my thinking about the practice of security: ratchets and levers.

Ratchets

A ratchet is a kind of one-way gear, with angled teeth and a pawl that allows motion in one direction only. In the physical world we use ratchets to help lift or move heavy loads. Using a ratchet, we can overcome the massive inertia of a heavy object by breaking the movement down into small, easy, irreversible steps.