You need rules? For word association? Okay, here goes for some arbitary rules then...
1) you can reply to any thread, including those that have already been replied to.
2) it may help to put the word you're responding to in the subject, or maybe not.
3) for more bizarre responses you can put your reasoning, if you want (and if you know).
4) NEW RULE! I forgot about threading - use the subject line to do the word association thing, and then just put any old nonsense in the body - so that people can easily see the latest association when it gets too long
p.s. Shall have to do something about my friends list soon - it's currently at 504, and I'm not sure how (or who's on it), as I can't even view it as my account type apparently doesn't support being able to see it, so I can't even edit it without an external client. Currently favouring getting rid of everyone and starting from scratch.