Basically, you make a wish. Then, the person who posts after you grants the wish, but in some cruel way, then THEY make a wish. Wash, rinse, repeat:
Poster 1: I wish I had a tall glass of water so I won't be thirsty anymore.
Poster 2: Granted, but the water completely dehydrates you.
I wish I didn't have a headache.
Poster 1 (or Poster 3 in case someone else posted first): Granted, now you have a migraine instead.
I wish *insert random wish here*
...and so forth and so on. Now to start:
I wish I had an unlimited supply of Welch's strawberry soda.