The main part of communication is content, and the quality of the content is the main element that makes it useful.
If the quality of your posts is high, you go a long way towards getting what you want from your message. If the quality of your posts is usually low, people will slowly start skipping your messages, even if (and especially if) you post very often.
Ensure you are adding useful information
Make sure that every single one of your posts contributes at least one piece of missing useful information.
For example, "RTFM" without a precise pointer is not useful information: if someone already knew where to RTFM, they wouldn't bother to ask.
Try also not to repeat your points: when you have a good argument, idea or claim, let it be heard once. Only bring it up again if you can come up with code, specifications or technical explanations to back it up.
Share the help you receive
Solving a problem creates knowledge, and it is important to share it. A useful IRC or email conversation can become a blog entry, a wiki page, a short tutorial or HOWTO.
When you receive help, try to take notes of all it actually takes you to completely solve the problem, and share them.
Reuse existing resources
When you are asking a question, do some research before posting: not only finding something on Google will give you an immediate answer, but it will avoid making people upset by asking the same question over and over again.
Often you may end up reading a list archive with mails related to your problems. Follow the threads to learn more from the experiences of other people.
If you see an error message you don't understand, pasting it in the Google search field often gives you useful information.
If you are replying to a question instead, a bit of research will allow you to provide a more precise answer.
When you find useful informations, post pointers to existing resources.
If a problem has already been solved or a point has already been made, post a link to the existing resource rather than repeating previous arguments.
If the existing resource is not good enough, try to improve it: either in place (for example, if it is in a wiki) or by posting your further comments together with the link.
Know what you want and make sure people know what they want
The biggest part in getting a good answer is to make a good question: try to work out what is that you really want. Ask yourself what are you really trying to get, where is that you are stuck, what were you expecting that did not happen. If you are lost, try asking yourself these four questions (this is called "Flanagan's Critical Incident Technique"):
What led up to the situation?
What did you do that was especially effective or ineffective?
What was the outcome or result of this action?
Why was this action effective, or what more effective action might have been expected?
The same questions are also useful to help clarify a situation when the other person is confused, and can be very handy when replying to a request for help or a bug report.