Not as easy as it may seem. Few rules: no random words; your message must be at least coherent, and should basically reflect what you think of that person or how they're viewed. You can use prepositions if you want, so someone with a username incorporating the letters i, a and b could become Is A Bastard or whatever. Also, try to keep it clean.
Example: mine would probably be something like:
Touchy
Reliable
Oversensitive
and so on; I'm not going to do me. Any guesses on who I
am gonna do?
Voluptuous
Amazing
Never
Insulting
Lovely
Lusted
After
And that's how you do it...