Here are some great (IMHO) security principles that Peter Gregory has in Chapter 2 of his book "Solaris Security."
They work for any type of system, not just Unix or solaris. (NOTE: There are nine of them. I'm just showing my favorites.)

Principle 1: The Hacker Who Breaks into Your System Will Probably be Someone You Know.
Principle 2: Trust No One, Or Be Careful About Whom You Are Required to Trust. (see rm -rf /)
Principle 2a: Don't Trust Yourself, or Verify Everything You Do. (see sysadmin horror story)
Principle 8: Before Connecting, Understand and Secure.
Principle 9: Prepare for the Worst. (see backup and offsite backup)

"The nine security principles discussed in this chapter represent a new way of (life) thinking about systems and security."

I think they can go far beyond just computers...