Here are some general 'rules' regarding posting messages.
Keep to the topic of the newsgroup. There are newsgroups for just about every conceivable interest, so at least make sure you post to the correct ones. If you feel an off topic post is necessary, then at least mark it as 'OT' in the subject. You don't want to be one of those annoying people that ask 'Where can I get Amiga ROMs?' on comp.sys.amiga.emulations...
Read the FAQ. Many newsgroups have a FAQ (Frequently Asked Questions) which is posted periodically (usually every two weeks or so) which includes information such as what it on-topic for the newsgroup, and answers to commonly asked questions on that group - so it is a good idea to read it before asking a generalised question. Note that FAQs are often considered "unofficial" and some newsgroups may have more than one FAQ. If you want to know exactly what is and what isn't allowed in the group, you should seek out the Charter (try searching on http://groups.google.com ).
Only quote those parts which are relevant to what you are saying when replying or following-up a message. Don't write your message and then leave the entire message quoted below! This is just a waste of bandwidth.
Limit signatures to a maximum of four lines of eighty characters. Again, longer .sigs just take up space and waste bandwidth.
Do not send binaries to text-only newsgroups This is a complete waste for the many people who will not wish to receive whatever you are sending. Many people have to pay by the minute for their 'phone calls, and will not appreciate spam. If people want to receive a binary, either email it to them individually, or place it on a website and post the URL to the newsgroup.
Stick to the correct language of the newsgroup. In many cases, this will be English, but not always.
Do not crosspost to a large number of newsgroups. If you feel that your message is appropriate in several newsgroups, then set a 'Followup-To' so that the replies only go to one newsgroup.
In general, don't ask for private replies.. Don't ask for private replies simply because you can't be bothered to read the newsgroup. There may be other people with a similar question who would like to read the replies, and it defeats the whole point of a general discussion group. If you expect people to take the time to answer your message, you should take the time to read the newsgroup. There are exceptions to this (in particular, buying/selling newsgroups), and in this case, you should type 'poster' into the 'Followup-To' gadget when typing your message. When following up a message that has 'Followup-To: poster' in the header, you will be warned that the poster intending email replies, but you may decide to post to the newsgroup if you feel that it is appropriate (for the reasons I have given). A possible compromise is to use the 'Reply & Followup' menu option so that it goes to the newsgroup, and to the poster via email.
Don't generally use capitals. Capitals are taken to mean shouting, so only use them unless you need to. Emphasis can be created by putting words in asterisks (*) which is taken to mean bold. Similarly, use slashes (/) for italics, underscores (_) for underlining, and hashes (#) for coloured. NewsCoaster interprets all of these when displaying messages (unless in Plain Text mode).
It is difficult to convey meaning across emails and news postings, in particular sarcasm could unnoticed. Use smileys such as ':)' to help avoid this - or maybe ';)' to convey irony.