Leroy Jethro Gibbs has rules that he enforces with everyone on his team, in 3x05 "Switch" he told Ziva that there are approximately fifty rules. However, only a few rules have been revealed. Here are the rules that have been revealed so far:

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 #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'

FULL LIST AT link