If etiquette is the grease on the axle of society, then netiquette is the grease on the axle of a mailing list. As a list owner, you need to make sure that your list's members are behaving themselves both socially and technically.
On the technical front, it's worth including a set of rules in your FAQ about the format of posts to the list. Suggested rules include:
No binaries or other attachments
No HTML or other non-text formatted mail
Wrap text at under 80 characters
Signature files no more than 4 lines long
On the social front, you might like to suggest:
NO SHOUTING
Keep it nice -- no name-calling, insults, or offensive material
Remember that it's hard to tell a person's tone of voice in email. Ask yourself whether a post could have been intended as a joke, sarcasm, or a hypothetical idea before flaming.
Don't repost private mail to the list; don't repost list mail to anywhere else
Before posting a question, check the FAQ, documentation, and recent posts to the list to see if your question has already been answered