Honestly...
Don't be too random, stay on topic.
Respect people's opinions. If you disagree with someone, write out your post calmly and be intelligent about it.
No bashing.
Don't make your signature too big and random.
Don't post about things you don't know. For example, if there's a thread about Global Warming, and you know nothing about it, then don't pretend you do.
Try your hardest to use proper puntuation and spelling.