The most important rule of netiquette is "Think before you post". If what you intend to post will not make a positive contribution to the newsgroup and be of interest to several readers, don't post it! Personal messages to one or two individuals should not be posted to newsgroups, use private e-mail instead.
When following up an article, quote the minimum necessary to give some context to your reply and be careful to attribute the quote to the right person. If the article you are responding to was posted to several groups, edit the distribution ("Newsgroups:") header to contain only those groups which are appropriate to your reply, especially if the original message was posted to one or more inappropriate groups in the first place.
Re-read and edit your posting carefully before you post. Check the spelling and grammar. Keep your lines to less than 70 characters. Don't post test messages (except to test groups) - wait until you have something to say. When posting humorous or sarcastic comments, it is conventional to append a smiley, but don't overuse them.
Before asking a question, read the messages already in the group and read the group's FAQ if it has one. When you do post a question, follow it with "please reply by mail and I will post a summary if requested" and make sure you DO post a summary if requested, or if only a few people were interested, send them a summary by mail. This avoids umpteen people posting the same answer to the group and umpteen others posting "me too"s.
If you believe someone has violated netiquette, send them a message by private e-mail; do not post a follow-up to the news. And be polite; they may not realise their mistake, they might be a beginner or may not even have been responsible for the "crime" -- their account may have been used by someone else or their address forged.
Be proud of your postings but don't post just to see your name in pixels. Remember: your future employer may be reading.
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This article (or an earlier version of it) contains material from FOLDOC, used with permission.