The Dalai Lama once said, "Learn the rules so you know how to break them properly," and while you don't want to break the rules about what NOT to do to get revenge on your ex-girlfriend, it does pay to KNOW those rules so that when you do set out to get revenge, you succeed in making your ex-girlfriend jealous rather than making her call the cops, feel sorry for you, or thank God she dumped you when she did.
1.
Don't Do Anything Punishable By Law Let's just go ahead and get this out of the way: Don't do anything that could land your tail in court, on the other side of a restraining order, or behind bars.
Most women do not screw around when it comes to annoying or possibly dangerous ex-boyfriends, and if your ex-girlfriend feels threatened enough (or ticked off enough), you might just wind up on the next episode of COPS.
Most of these kinds of stupid revenge tactics are pretty obvious, but in case you need brought up to speed, do not:

• Cause physical harm to her or anything that belongs to her.
  This obviously includes physically touching her in any way, but it also includes ignorant moves like breaking into her home and trashing or stealing anything that belongs to her, slashing her tires or busting out her windshield, grabbing her cat when she leaves for work and taking it to the pound, etcetera and so forth.
  
• Do anything that a judge would consider harassment.
  This includes creeping past her apartment 20 times a day, prank calling her all hours of the night, taking out advertisements or renting billboards declaring what a whore she is, cutting letters out of magazines and crafting a "ransom"-style note professing your hatred for her, calling her boss's wife just to let her know your ex is sleeping with her husband - you get the idea.
  

As a side note, it'd be a great idea not to hire anyone to do these things for you, either.
Actually, as another side note, it'd be a great idea to seek some professional help if you think any of these sounded perfectly reasonable.
2.
Don't Do Anything That Screams "I'm Pathetic!" In addition to avoiding everything that ultimately risks your butt's virginity, also take care to avoid anything that makes you look desperate.
Nearly every guy dreams of getting revenge after he's been dumped; yet, judging by my own experiences, nearly every guy also has no idea how to really get revenge on an ex-girlfriend.
Actually, most times the guy starts out with a great idea but screws it up somewhere along the way.
In other words, if you want to make your ex-girlfriend jealous:

• Rebound relationships usually work, but if you're rebounding with fives and your ex-girlfriend was a nine, you might as well call her up and admit you'll never do any better than her.
  
• Getting yourself a social life usually works, but if you're hitting the clubs so often you're making Amy Winehouse look responsible, the only thing your ex-girlfriend is going to feel is sorry for you.
  She might even stage an intervention.
  

The key is to find a balance.
For example, your rebound chick doesn't have to be a Playboy Bunny, but she does have to have something your ex-girlfriend doesn't - brains, money, an ass only God himself could've crafted - whatever it is your ex lacks, and knows she lacks.
3.
Don't Do Anything That Shows You've Stayed the Same Actually, that should probably just read "Don't Stay the Same.
" Sounds harsh, yes, but really - what were you expecting? A big hug? Someone to assure you you're perfect just the way you are? Yeah, save that crap for your mom.
In reality, you might be one hell of a guy, but there are two hard facts in this situation:

1. Your girlfriend dumped you.
   
2. She dumped you for a reason.
   

If you really want to make your ex-girlfriend jealous, you have to change that part about yourself that made her dump you.
If it was your job, start looking for something outside of the fast-food industry.
If it was your lack of social grace, start paying attention to those guys who seem to be the life of every party they attend.
If it was your beer gut, trade your six-pack of Bud for a six-pack of steel.
The goal is to show her there's more to you than she thought and that maybe, just maybe, she jumped the gun a little too soon.