I wrote these five guidelines in a few minutes so they can surely be improved. :) Make your suggestions, also about points I might have forgotten.
Some guidelines on how to write a proper post (answer or question):
Always include context, details, background. There is a question body so you can elaborate on the "title/question" in order to be more specific and clear about what you're asking. Providing info, not only reduces the risk of down-voting, but it also improves the chances of having a proper answer. The better the question, the better the answers!
Write appropriately and use the correct formatting. This means use capitals letters and punctuation when it's required, also use the blockquotes for quotes and not the code-field, which exists to write code. A well-written question/answer is one of the steps of attracting better answers or upvotes too. Also, avoid writing a huge single paragraph. Five small paragraphs are more readable than a huge single one.
Don't write links like "this" or "here". Choose a wording where the text-link is the title of the document you're linking; specify when it's an image, for those who have slow connections.
If possible, include a reference. Stackexchange sites are about definite, objective answers and not about opinions, (that's why we have the "subjective question" close vote), so providing reference, you're assuring the OP and the readers that what you say is true and backed up.
Avoid one-line answers. It might be correct, but an answer made by a single sentence is not that informative. A bit of elaboration is very useful for the reader.
Be kind to other users. If you're writing a question, remember that you're asking to be helped, so make sure to show your appreciation to the ones who answer by being kind. If you're answering, instead, don't attack the OP and don't be aggressive. It won't bring any good to the site. Like they say "Treat others as you wish to be treated".
Last, but not least, look at this page on how to ask a question properly: "Writing the perfect question".
Ideas? Questions? Doubts?