If you've played the Republic Commando game, you've probably heard Scorch or Sev quote "rules", for example: Rule 17: Always make sure they're dead. I've gathered all the rules I could find and presented them in order below. Note that some of these aren't in the final game and are only mentioned in the dialogue subtitles.
Rule 1: Kill 'em before they kill you.
Rule 17: Always make sure they're dead.
Rule 23: Never pull sentry duty on an empty stomach.
Rule 39: Never say no to bacta.
Rule 45: Any crash you can walk away from is a good one.
Rule 89: One's an anomaly, two's a trend.