I try to follow several of these religiously.

Gibb's Rules
Rule #1: Never let suspects stay together. (Yankee White, 1.01)

Rule #1: Never screw over your partner. (Blowback, 4.14)

Rule #2: Always wear gloves at a crime scene. (Yankee White, 1.01)

Rule #2: Unknown at this time

Rule #3: Don't believe what you're told. Double check. (Yankee White, 1.01)

Rule #3: Never be unreachable. (Deception, 3.13)

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. (Blowback, 4.14)

Rule #6: Never apologize. It's a sign of weakness. (Specifically mentioned in Flesh and Blood, 7.12)

Rule #7: Always be specific when you lie. (Reveille, 1.23)

Rule #8: Never take anything for granted. (Probie, 3.10)

Rule #9: Never go anywhere without a knife. (One Shot, One Kill, 1.13 ; Missing, 1.20) Sometimes as "Never leave home without a knife."

Rule #10: Never get personally involved in a case (Obsession, 7.21)

Rule #11: When the job is done, walk away. (Semper Fidelis, 6.24)

Rule #12: Never date a coworker. (Enigma, 1.15)

Rule #13: Never, ever involve a lawyer. (Rule Fifty-one, 7.24; Collateral Damage, 6.07)

Rule #15: Always work as a team. (Leap of Faith, 5.05)

Rule #18: It's better to seek forgiveness than ask permission. (Silver War, 3.04)

Rule #22: Never, ever bother Gibbs in interrogation. (Smoked, 4.10)

Rule #23: Never mess with a Marine's coffee if you want to live. (Forced Entry, 2.09)

Rule #27: There are two ways to follow someone. -- First way they never notice you. -- Second way they only notice you. (Rule Fifty-one, 7.24; (Jack Knife, 7.15)

Rule #38: Your case, your lead. (Bounce, 6.16)

Rule #39: There is no such thing as coincidence. (Obsession, 7.21)

Rule #40: If it seems someone is out to get you, they are. (Borderland, 7.22)

Rule #44: First things first, hide the women and children. (Rule Fifty-one, first mentioned in 7.23)

Rule #45: Clean up the mess that you make. (Rule Fifty-one, 7.24)

Rule #51: Sometimes - You're wrong (Rule Fifty-one, 7.24)

Rules according to Di'Nozzo:
#3. Never Underestimate your opponent [s03e24-"Hiatus 2"]

Rules according Abby:
#9. Always keep a spare. [s03e22-"Jeopardy"]