Rule #1: Never let suspects stay together.
Ep 101 "Yankee White"

Rule #1: Never screw over your partner.
Ep 414 "Blowback"

Rule #2: Always wear gloves at a crime scene.
Ep 101 "Yankee White"

Rule #3: Don't believe what you're told. Double check.
Ep 101 "Yankee White"

Rule #3: Never be unreachable.
Ep 313 "Deception"

Rule #4: The best way to keep a secret? Keep it to yourself. Second best? Tell one other person - if you must. There is no third best.
Ep 414 "Blowback"

Rule #7: Always be specific when you lie.
Ep 123 "Reveille"

Rule #8: Never take anything for granted.
Ep 310 "Probie"

Rule #9: Never go anywhere without a knife.
Ep 113 "One Shot, One Kill" and Ep 120 "Missing"

Rule #12: Never date a coworker.
Ep 115 "Enigma"

Rule #15: Always work as a team.
Ep 505 "Leap of Faith"

Rule #18: It's better to seek forgiveness than ask permission.
Ep 304 "Silver War"

Rule #22: Never, ever bother Gibbs in interrogation.
Ep 410 "Smoked"

Rule #23: Never mess with a Marine's coffee if you want to live.
Ep 209 "Forced Entry"