Civilized Discussion Guidelines

Debate ideas, not persons.
Make a clear distinction between objective information and subjective information.
Avoid "you always do X" without specifically listing at least 3 specific situations.
Avoid AdVerecundiam arguments if possible.
Avoid "guessing" a person's negative motive(s) for having a certain opinion.
AgreeToDisagree when appropriate without hard feelings.

The hard part is what to do if somebody violates these.

If appropriate, politely point out their violation.
Whatever you do, don't respond in kind (this might start a HolyWar)

Alternative Guidance

Recognize that it is impossible to change another person's belief; that person may or may not choose to change his beliefs. Everyone has a different set of knowledge and experience and ones beliefs will be base on these underlying elements. People with different sets of knowledge and experience can logically come to different conclusions based on the same evidence.

When another challenges a statement, use that as a chance to understand one's own beliefs. Try to justify why one believes something to be true, not in order to prove the other person wrong, but to understand why one believes it to be true. Realize, also, there will come a point where one reaches his fundamental beliefs and one will not be able to justify a statement. Have the courage to state "I cannot justify this statement, I merely believe it to be true." At this point, one should walk away from the discussion, one can only repeat what has already been said, and it is unlikely that one will find any deeper understanding.

Finally, when one walks away, accept that one may not have the last word in the discussion. Leave any further questions unanswered, one has no more to say on the subject at this time.

Assume others will follow this approach. Misread any accusations as merely questions and feel free to edit the accusations into answerable questions. More often than not, replying in a dispassionate manner will raise the level of discussion. Also recognize that everyone has a bad day where he might state something in a poor manner. Look for the underlying logic in the statement and ignore the emotional wording. Feel free to edit emotional wording to raise the level of discussion.

For a style aimed at mutual agreement, see PositiveDialogueCommunity

Suggested Civility Guidelines:

Remember, people take offence at different things.

Try not to assume the worse when a post makes you hurt or annoyed.

Keep a sense of humor.

Consider others may hold opinions which are dramatically different from your own.

Consider you might be wrong.

Consider there might be some truth in what the other person is saying.

Wait a few hours before responding to a post which has upset you.

You are as much responsible for your reaction as the supposed offender is responsible for the offence.

Use "I" messages when responding.

Consider how your words might be received.

Consider whether you'd say your message face-to-face.

The offended party decides what is offensive.

Apologies are best; however, if you wish not to apologize, consider clarifying your statement to better explain the intent behind an offending post.

Be as direct as possible when responding to someone who has been offended by your post.

When offended, write off-list to the person saying, `When you said xxx I understood yyy. Is that what you meant?'

Write off-list to the moderator(s) explaining how you felt/perceived the message, and ask the moderator to deal with it off-list.

If all else fails, attempt to ignore someone (delete their posts unread) who is especially irritating you.