Well, here you are stepping off the Midlife Express in El Dumpsville. Whoever you were just with got tired of putting up with your guff and 86ed you right out of the nest. Well, as long as you're personna non gratia in your ex's life, why not go all the way? Being on the outs is not enough, now you want to make your ex hate you.
Well, if you want to get back together, this is a list of things NOT to do. On the other hand, if you think that separation is the best thing for you but you fear that at some point in the future your resolve will melt and you'll be back to the old ball 'n' chain, then you might find some of these to your advantage.
Calling several times a day is the biggest step. Fill their voice mail, leave messages at their work... leave messages with their new mate! How's that for making the new person feel welcome, using them for nothing more than a messenger? If you call their job enough, they might get fired, but at the least you'll get their attention. If they block your number, well, that's what disposable cell phones are for.
While you're at it, don't neglect email, text messages, and good old fashioned letters! Maybe even stalk them around town, watching from a distance as they go about their business. If you're spotted, just grin and wave back! All you have to do is push this until you're actually served with a restraining order, and then you can sit back and relax, because your point has been made. Don't worry about your legal record; these days they hand out restraining orders like Christmas cards, and they're taken just about as seriously.
Keep up a relationship with your ex's parents. Try to be their best friend! Ooooh, how much is it going to burn their bacon when mom and dad bring it up in conversation again and again what a swell turnip you were! That will make your ex hate you even more!
Failing that, try dating one of your ex's best friends! Whatever you can do to keep your face in their face, and their best friend will try to be cool about it, but the tension will drive them apart like a wall of obsidian and then you can casually dump them for extra evil points.
Pick up a new belief system that involves doing someone else in. Whether it's Kaballah and you're forming a clay golem to chase them through their nightmares, voodoo and you're stabbing their wax doll effigy with rusty hatpins, or good old fashioned Satanism and you're summoning a demon to possess your ex's house with a cupboard-rattling poltergeist, practicing some form of black magic will keep your hate focused.