I've seen this on various forums and it was/is very popular, so I thought I'd post it here. All you do is what the title of the thread says. Here's an example:
FurryLover is above me so I post FurryPuff
Then Furry4Life sees my post and posts Fuchsia4Life
Then CatWhisperer sees Furry4Life's post and posts FurryWhisperer
And the cycle continues.
I'll start:
NoUserPuff