Stolen from several websites (the one where I first encountered it was the forum for Casey & Andy)....
Step 1: Corrupt the wish of the above poster.
Step 2: Make your own wish.
Ex.: The above poster wishes for a pony.
NextPoster1: Granted, but it's an evil flesh-eating pony. I wish I had superpowers.
NextPoster2: Granted, but the powers drain your life-force, causing you to age a an accelerated rate. I wish for $1,000,000.
NextPoster3: Granted, but your credit cards are stolen, and you spend the million paying back fraudulent charges. I wish I was ruler of the universe.
Starting with an easy one - I wish that I had my very own space station!
The Auld Grump