Don't back-stab your friend.
Don't black-mail your friend, even if she becomes an EX-friend.
Don't talk about your friend [badly] behind her back, unless it's just like a minor issue that you want to let off your shoulders.
Don't go out with your friend's brother unless she is fine with it.
Don't b/tch at her because you're having a bad day. Because you'll end up feeling like a total dumba-ss afterwards when begging for forgiveness.
Don't choose a relationship over a true friendship, unless the friend is a back-stabbing b/tch who made your life miserable and only pretended to be your friend for your money and popularity.
There are many more, but those are the most general ones. Oh and the most important one of ALL!
Don't have sex with your friend's husband, boyfriend, brother, sister, ex-boyfriend, ex-husband, fiance, fiancee, and other relatives who are/were dear to her heart without her permission.
As long as you stick with the last rule, the other ones are easier to manage. :]
Lol, I hope some of you got a laugh out of those. So true, but so harsh, I know. ;)