think this part is very important:
Use meaningful, specific subject headers
On mailing lists, newsgroups or Web forums, the subject header is your golden opportunity to attract qualified experts' attention in around 50 characters or fewer. Don't waste it on babble like “Please help me” (let alone “PLEASE HELP ME!!!!”; messages with subjects like that get discarded by reflex). Don't try to impress us with the depth of your anguish; use the space for a super-concise problem description instead.
One good convention for subject headers, used by many tech support organizations, is “object - deviation”. The “object” part specifies what thing or group of things is having a problem, and the “deviation” part describes the deviation from expected behavior.
Stupid:
HELP! Video doesn't work properly on my laptop!
Smart:
X.org 4.1 misshapen mouse cursor, Fooware MV1005 vid. chipset
Smarter:
X.org 4.1 mouse cursor on Fooware MV1005 vid. chipset - is misshapen
The process of writing an “object-deviation” description will help you organize your thinking about the problem in more detail. What is affected? Just the mouse cursor or other graphics too? Is this specific to the X.org version of X? To version 4.1? Is this specific to Fooware video chipsets? To model MV1005? A hacker who sees the result can immediately understand what it is that you are having a problem with and the problem you are having, at a glance.
More generally, imagine looking at the index of an archive of questions, with just the subject lines showing. Make your subject line reflect your question well enough that the next guy searching the archive with a question similar to yours will be able to follow the thread to an answer rather than posting the question again.
If you ask a question in a reply, be sure to change the subject line to indicate that you're asking a question. A Subject line that looks like “Re: test” or “Re: new bug” is less likely to attract useful amounts of attention. Also, pare quotation of previous messages to the minimum consistent with cluing in new readers.
Do not simply hit reply to a list message in order to start an entirely new thread. This will limit your audience. Some mail readers, like mutt, allow the user to sort by thread and then hide messages in a thread by folding the thread. Folks who do that will never see your message.
Changing the subject is not sufficient. Mutt, and probably other mail readers, looks at other information in the e-mail's headers to assign it to a thread, not the subject line. Instead start an entirely new e-mail.
On Web forums the rules of good practice are slightly different, because messages are usually much more tightly bound to specific discussion threads and often invisible outside those threads. Changing the subject when asking a question in reply is not essential. Not all forums even allow separate subject lines on replies, and nearly nobody reads them when they do. However, asking a question in a reply is a dubious practice in itself, because it will only be seen by those who are watching this thread. So, unless you are sure you want to ask only the people currently active in the thread, start a new one.