At the Ágiles2009 coding dojo,  instead of trying to explain the whole concept of coding dojo, I jumped straight to a quick list of items that attendees should bear in mind during the session:

• If you know how to code, you should code
• If you're coding, everyone else has to understand what you're doing
• If you're the next to code, avoid breaking the flow
• If you're not coding, don't disturb who is
• If you have an idea, show it with code
• If you're stuck, ask for help
• If you liked the challenge, try it again at home

Interesting how this simple "etiquette" helped to make what I consider one of the best dojos I've participated so far.