Before I begin my tirade of the day, I'd like to just say I do not feel this way about all my exes, I am not just talking about just MY exes and this is not limited to heterosexual relationships. Also, if you know someone who is truly suicidal, send the cops to their house and that's all you should do.
GUILT TRIPS: They can't believe the cold way you broke up with them whether it was a letter or whatever. Except for the people who are truly callous aholes, they probably broke up with you that way to avoid your drama. They are not required to put up with your melodramamtic break up scene because they were probably putting up with it a lot before and are sick of it, so you should suck it up and STFU.
SUICIDE THREATS: Picking up guns and putting them to different parts of your body are not going to endear us and make us reconsider. Depending how difficult the relationship was, they are probably halfway wishing you'd do it. Hey, don't shoot the messenger.
DRUNK DIALING: Unless the person you were dating is a total idiot, they are not going to find this charming. In fact, they will at least change their number and probably tell all their friends what a pathetic loser you are. If I were you, I'd consider this behavior suspect of an addiction and find an AA meeting.
DELUSIONAL THINKING: Apparently by breaking up, this person is giving you the message that they don't want to be with you. Do not show up at their house late and night with a cool hat on and try to kiss them. While the hat may be cool to some people, it is not going to disguise the fact that you are some sort of mistake to that person and your stupidity is really pissing them off.
CRYING: Keep that to yourself or I'll give you something to cry about.
ASKING AROUND: Do not ask everybody, including this person's friends and family members how this person is, where they live, work or to pass on a compliment. If you wanna know that bad, have the guts to ask them and let them not answer.
THE ONE NIGHT STAND: This person intended to hit it and quit it. Don't show up at their house pathetically like you really had something together. Go find somebody else. Really.
STARING: For those of you socially retarded people, staring at someone who broke up with you (in all likeliness because they finally wisened up) is going to make them uncomfortable. STALKER.
ACCUSING THEM OF CHEATING: You probably suck in bed, but that doesn't mean they were cheating on you. Certainly they are willing to knaw off their own arm just to get away from you to see somebody new....or not. Just saying. Their desire to be away from you has nothing whatsoever to do with character and fidelity.
SLANDERING: It's just a bad idea in the grand scheme of life. Everyone who knows them knows you're a bitter reject and that stuff is probably not true. Oh, and they may say something equally bad about you and everyone will KNOW that's true.
All this works on women and men who don't have any sense, but not, say, ME for example. So if I see you out on a ledge somewhere, I may be thinking let it happen JUST THIS ONCE, until, of course, I'm not paying attention...