Rule #1: Never let suspects stay together.
              (Franks) 1x01 'Yankee White'
Rule #1: Never screw over your partner. 4x14 'Blowback'
Rule #2: Always wear gloves at a crime scene.
              (Franks) 1x01 'Yankee White'
Rule #3: Don't believe what you're told. Double check.
              (Franks) 1x01 'Yankee White'
Rule #3: Never be unreachable. 3x13 '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. 4x11 'Blowback'
Rule #5: You don't waste good. (…"You're Good" Gibbs says to DiNozzo) 8x22 'Baltimore'
Rule #6: Never apologize — Its a sign of weakness.
A rule used more often than the others, but finally given a number in 7x12 'Flesh and Blood' during a great Gibbs/DiNozzo moment. The first person to use that phrase was John Wayne in, "She Wore a Yellow Ribbon." "Never apologize, mister, it. It's a sign of weakness." — In the episode 3x23 'Hiatus Part-1' DiNozzo explains to NCIS Director, Jenny Shepard, that Gibbs took this one from "The Duke himself."
Rule #7: Always be specific when you lie. 1x23 'Reveille'
Rule #8: Never take anything for granted. 3x10 'Probie'
Rule #9: Never go anywhere without a knife. 1x13 'One Shot, One Kill' & 1x20 'Missing'
Rule #10: Never get personally involved on a case. (Said by Tony, agreed on by Gibbs) 7x21 'Obsession'
Rule #11: When the job is done, walk away. 6x24 'Semper Fidelis'
Rule #12: Never date a coworker. 1x15 'Enigma'
Rule #13: Never, ever involve a lawyer. 6x07 'Collateral Damage' & 7x24 'Rule Fifty-One'
Rule #15: Always work as a team. 5x05 'Leap of Faith'
Rule #16: If someone thinks they have the upper-hand,
…break it! (DiNozzo) 8x24 'Pyramid'
Rule #18: It's better to seek forgiveness than ask permission. 3x04 'Silver War'

Rule #22: Never, ever bother Gibbs in interrogation. 4x10 'Smoked'

Rule #23: Never mess with a Marine's coffee if you want to live. 2x09 'Forced Entry'
Rule #27: Two ways to follow: First way, they never notice you, — second way, they only notice you. 7x15 'Jack Knife' & 7x24 'Rule Fifty-One'
Rule #35: Always watch the watchers. 8x22 'Baltimore'
Rule #38: Your case, your lead. 6x16 'Bounce'
Rule #39: There is no such thing as coincidence. 7x21 'Obsession'
"The 40's are for Emergency Use Only!"
(Mentioned by Abby) 7x24 'Rule Fifty-One'
Rule #40: If it seems someone is out to get you, they are. (Said by Abby, agreed on by Gibbs) 7x22 'Borderland' & 7x24 'Rule Fifty-One'
Rule #44: First things first. Hide the women and children. 7x23 'Patriot Down'
Rule #45: Clean up your own mess. (Gibbs Said, "Left a mess I've got to clean up." — DiNozzo commented that's basically Rule #45) 7x24 'Rule Fifty-One' (See Below)
Rule #51: Sometimes — You're Wrong! 7x24 'Rule Fifty-One'
And remember…
 
"A slap to the face is an insult — to the back of the head is a wake-up call."