Never hate a woman you have never met.
Never date a friend's ex. Unless you don't want her as your friend.
Never reveal another female's secret.
Never leave an inebriated friend alone at a bar.
Never invite a friend's enemy to the party. You gotta stay together to survive the war :-).
Never dine alone with a friend's boyfriend or husband. Unless she is dead.
Never stay quiet when a friend is falling for an asshole. You will be cleaning the mess later anyways.
Never trust a girlfriend who dates a married man or never trust a girlfriend who is having an extra-marital affair. There are simply no excuses.
Never give constructive criticism in public. Give it to her in person. And most importantly -
Never treat other women disrespectfully. It gives men ideas.