For every forum you click on, the rules of the forum should flash up at the bottom right or left of your screen. I.e., where the wasted grey areas are. Say, the feedback thread posts up what NOT to post and what is rejected and will forrever be rejected. Too many people keep posting things that will never come to be because they don't manually find the rules. It's annoying seeing new threads, yet they always post the same "additions" over and over again.
I work on a gaming website and have it so the rules flash up in the bottom right next to our arrowchat. It works... if only sometimes.
Maybe if it had the text above the typing field, as recommended above, and in that text it had links to rejected ideas, rules etc.? Otherwise it might end up with a pretty big rule box in some cases.
But yeah whatever form it may take, I think it's a good idea