Gibbs' Rules:
#1 Never let suspects stay together.
#1 Never screw over your partner.
#2 Always wear gloves at a crime scene.
#3 Don't believe what you're told. Always doulbe check.
#3 Never be unreachable (Episode Deception)
#4 If you have a secret, the best thing is to keep it to yourself. 
#7 Always be specific when you lie.
#8 Never take anything for granted
#9 Never go anywhere without a knife.
#11 When a case is finished let it go.
#12 Never date a coworker.
#13 Never involve lawyers.
#15 Always work as a team.
#18 It's better to seek forgiveness than ask permission.
#22 Never ever bother Gibbs in interrogation.
#23 Never mess with a Marine's coffe.
#26 Never apologize - it's a sign of weakness.
#38 Your case, your lead.

A THANKS TO DAN REID AND OTHER MEMBERS FOR HELPING UPDATE THE RULES!!!