There are many ways to seek revenge, but the most effective is the kind that results in your "victim" not only learning to never do what they did again, but why they shouldn't do it. That's why the best revenge is a mirror - so your target can see exactly how and why they're being an enormous douche.
When you're plotting revenge (or doing anything, really), you should have a point. Generally that point is best made by demonstrating to the offender why what they did was wrong and what to expect from you should they do it again. For example, if someone's dog takes a dump on your lawn and they don't clean it up, you can put it in an envelope with a nice note that reads "I noticed you left this on my property and just wanted to do the neighborly thing and return it." Just make sure it fits nicely in the mailbox.
If a friend or family member is always late, even when it really matters, wait until they invite you to something important and then show up long after you were supposed to. When someone's talking loudly on the phone in a movie theatre, go sit next to them (or stand if you can't) and start talking into their ear.
Obviously these things mean 1) you're asking for trouble, and 2) you're going to hurt some feelings (or at least piss some people off). Nonetheless, sometimes you need a little tough love to get the point across. If you have a problematic situation, sometimes the most effective way to approach it is to shine a mirror in the face of the offender. Sometimes it isn't, but it's a good concept to have in your bag of tricks for when the right circumstance presents itself.
Have you ever used a "douchebag mirror" strategy to get revenge? Share your story in the comments.