As an example, Tim Ferris has this right above the comment submission box (getting to users right when they're about to take an action):

  Comment Rules: Remember what Fonzie was like? Cool. That’s 
  how we’re gonna be — cool. Critical is fine, but if you’re 
  rude, we’ll delete your stuff. Please do not put your URL in 
  the comment text and please use your PERSONAL name or 
  initials and not your business name, as the latter comes off 
  like spam. Have fun and thanks for adding to the 
  conversation! (Thanks to Brian Oberkirch for the inspiration).

Screenshot: http://cl.ly/Ipex Guidelines are nice, but most people won't see them. It might work better to get to them right when they're taking the action.