In order to get a good answer from any site, you need to ask it clearly. Here are some keys:

1. Use a subject line that will get the attention of someone likely to have a good answer. It should be more specific than the category you specify: Don't say "Math help" on a math site, or "Algebra" in the Algebra category, or "Help please!!!" anywhere, which tell us nothing; rather, say something like "Equation of a line" or "Rate problem" or "How can I factor a cubic?". Briefly summarize the nature of your question.
2. Don't omit important information from the body of the question and put it only in the subject line; sometimes we forget to read the subject. In particular, don't start a sentence in the subject and continue it in the body. Similarly, do not include essential parts of the problem only in an attachment or picture without typing it in the body of the message.
3. State the entire problem, in detail. If it is an exercise or assignment you were given, quote the whole thing verbatim (including the choices in a multiple-choice problem); and if you are asking only about a late step in a multi-step problem, tell us the earlier steps and your answers, so we can see if something went wrong there. Also tell us the context -- what topic you are studying, or why you are asking.
4. Show whatever work you have done, or at least tell us what you know that might be of use. This gives us a starting point in guiding you to a solution using what you already know. It's important to understand that there are often several ways to approach a problem, and we will want to use the method you've been taught -- which we can't know unless you tell us. Also, tell us where you need help, and what went wrong.
5. Don't expect us to do your homework for you. Its purpose is to give you practice doing the math, not to see what someone else can do for you. (Would you go to gym class to watch a teacher do push-ups? No; it's the teacher's job to watch you do them, and give helpful advice.) In particular, we will not help you cheat.
6. Show that you have made an effort to learn; the more you say, the more effort we will put into helping you.
7. Keep a thread to one topic; if you have an unrelated question, don't tack it on to your current thread. This not only makes our conversations easier to follow, but also gives you a better chance of getting an answer, because other Math Doctors who are available are more likely to help with a new question. A question in an existing thread is likely to be looked at only by its "owner", who may not be available.
8. Don't submit many questions at once, without waiting for an answer to the first couple; you are likely to learn how to do the later questions from our answers to the earlier ones. Especially, don't ask the same question in more than one thread. If you want a different answer than we have given, tell us; someone else may join the discussion with a different perspective.
9. Ask your question in English; most of us don't know most other languages. But if you have to translate a problem into English, it may help to also give the original form, so we can check its meaning. Also, avoid text-speak; abbreviations like "d" for "the" or "ur" for "your" don't mix well with math symbols, and are likely to be misunderstood.
10. Format symbols in a way that can be read easily. We hope to support LaTeX formatting eventually, but don't worry if you don't even know what that is; see our article How to type math for standard ways to write formulas (such as "x^2" for x squared and "sqrt(x)" for the square root of x). Also, use parentheses to ensure that what you write means what you intend.
11. Be polite. Keep in mind that we are volunteers who are trying to help you. Help us to feel good about what we do. But if we goof, tell us (as gently as you can). We are also human!
12. Feel free to ask any questions you have about math, not just about particular problems! On the other hand, sometimes an example can make it clearer what you are asking about, so be specific when you can, even about a general question. See the blog for examples of interesting problems we have been asked in the past.

For more on writing clear questions, see http://www.purplemath.com/modules/mathtext4.htm.

For an expanded version of this article, see the post How to Ask a Good Question.