I can't tell you how to break up with your significant other. But I can tell you how not to do it. If you are over the age of 13, do not ever use instant messenger. Also never break up with someone mid-thrust. No evidence can completely dispel the potential for teeth in vaginas.
If you break up with someone over the phone, do not proceed to go over to their room to get laid one last time. Especially do not pretend to have changed your mind and re-dump them after you're done. Once you've said goodbye on the phone, you have had a 30-second period to call back and ask for sex.
You should not attempt to have breakup sex if it's clear the other person is far too emotionally distraught.
This means they'll definitely have sex with you, but because you didn't heed my advice they'll become clingier than a tick. And that's not Lyme disease you've contracted. You should also not have breakup sex if you're leaving your significant other for someone else that you've already started to sleep with. That's just greedy. Definitely don't have breakup sex if you're dumping them because they're lackluster in bed. That's just absurd.
You should have sex if you're breaking up in person, and the emotions are running high but not to the psycho-point. Do keep in mind the following guidelines:
This is not an opportunity to try something new, not the time to try that thing she never wanted to do because you won't have to deal with the repercussions.
Don't cry, as much as you want to. Break-up sex is angry, and that's what makes it good. Get as much of that anger out - the betrayal, the embarrassment, the sickened feeling you get when you look at him now - all of those things can constructively be transformed into really rough sex.
Consider not kissing. If you didn't try that in your relationship, it can actually really add something to the experience. It makes angry sex twice as angry and detached, and that's exactly what you should be looking for in breakup sex.
Break-up sex is a great opportunity for girls to reach orgasm and leave. Don't be tricked by the endorphins that tell you to stay and cuddle. You don't have to wait for him to climax - you've been catering to his needs long enough. Get up and leave. If you do both orgasm, leave soon afterwards. Do the thing you see in movies: Hang on to that happy feeling for 10 seconds, then start getting dressed. Leave. Even if you live together, leave.
As is the rule with any breakup, do not call the next day after break-up sex.
You'll sound six or seven times as desperate as you did during the breakup, and it should be clear that s/he isn't coming back. In case you weren't aware, the chances of them coming back increase if you are happy, single and carefree.
One last tip: Don't be afraid to go for the indie movie sex. Break up while sitting on opposite ends of a couch, and then have sex right there on said couch.
Don't worry about artfully tossing your clothes to the floor, that's for happy indie movie sex. Everyone should get a chance to have sex on a couch full of hate.